The Ten Commandments of Budget Deck Building by the Penny-Pinching Planeswalker
I began playing CCGs in 1996 with the Decipher's Star Wars CCG, and moved very quickly into Magic. Since then I have played roughly a dozen collectible/tradable card games (many of which no longer exist), always on a very limited budget. In my experience, Magic is definitely amongst the friendliest to those on a limited budget. A budget deck won't take you to the Pro Tour, but a properly-built one can give you a fighting chance even against tournament-caliber ones at your local game store and kitchen table. What follows is ten pieces of advice I have developed in my years of playing an expensive hobby with a miser's wallet: The Ten Commandments of Budget Deck Building.
I. Thou shalt know and accept thy limitations. Let's be perfectly clear - you will not win a Grand Prix with a $20 Standard deck. Budget decks can do many things - teach newer players the game, allow those heavily invested in one format to dabble in another, and serve as an entry point into unfamiliar formats - but they simply are not substitutes for highly-tuned (and high-cost) competitive decks. If you approach budget deck-building properly, however, you can compete in events such as Friday Night Magic and be able to defend yourself properly against decks that cost ten times more than yours. However, by running a budget deck, you're putting yourself at an disadvantage, because the power level of your deck is lower than the rest of the field. So you've got to be smart, not only in how you play it, but how you build it as well.
II. Thou shalt NOT build weaker versions of competitive decks. One of the easiest traps to fall into, particularly for those new to the hobby, is to net-deck a tournament-winning list, and substitute all the expensive cards you don't have for cheaper ones. As previously mentioned, you're already at a disadvantage by having a deck with a lower power level, but by making a weaker version of a known competitive deck, you're making your opponents' decks and sideboards super-effective against you! When a competitive player tuned his decklist, he did so keeping in mind the decks he would be playing against the most - like the deck that you took and replaced with weaker cards. Instead of fighting fire with fire, you're fighting fire with kindling.
III. Thou shalt know thy enemy, and thou shalt NOT fight fair. Don't fight fire with fire; fight fire with water balloons. Sure, water balloons aren't particularly powerful, but against a fire that's not ready for them, that's a fight you can win. Even though you're not going to build your deck based off a tournament-winner, you need to know the decks that are winning tournaments, so that you know specifically what not to build. The one advantage budget decks have is their roguish nature. In the pre-Gatecrash Standard, the player piloting a $600 Red-Black agro deck has built his deck and sideboard to handle other Red-Black aggro decks, Hexproof Enchantments, Bant control, Reanimator, and whatever else the field considers good. His deck wasn't fine-tuned to handle Mono-Black Control. Suddenly, you have a plan of attack - kill all his creatures and attack his hand until he's in top deck mode. Then cast your unfair triple-black casting cost spells for the win.
IV. Thou shalt prioritize spending. The most expensive part of your deck needs to be the spells that make the deck work. To this end, you want your mana base to be as inexpensive as possible. For most formats, this means sticking to one color. For Commander, this means using basic lands and cheap, "suboptimal lands." Unless you're playing competitively (in which case you may have missed the point of Commander), the value of a $90.00 Tundra provides over a 10-cent Azorius Guildgate in a 100-card deck is minimal. It is certainly possible to build multi-colored competitive decks with Evolving Wilds instead of Shocklands, but you're sacrificing speed and consistency - something you can't afford when you're already at a power disadvantage. So, just stick with one color, and spend the money in your budget on the cards you need to win games. Start with your own collection - the more cards you can use from it, the less you will have to acquire. This is especially true for Commander - if you got an expensive legendary creature in your prerelease pool, by all means build your deck around that card as your general.
V. Thou shalt NOT accept substitutions. If your Commander deck relies on token swarming to win, then you can't live without Doubling Season. Parallel Lives just doesn't do the job. You probably should have room for both in your deck, but if you only have room for one, spend the money on Doubling Season and cut costs elsewhere in the deck. If your mono red deck needs a Thundermaw Hellkite to be competitive, then spend the money on that Hellkite. If you can't, then you need to redesign your deck so it doesn't need Thundermaw Hellkite. You have to minimize your disadvantages, and having weaker versions of the cards you actually want in your deck puts you at greater disadvantage.
VI. Thou shalt make use of others' work. If you have no collection at all, then buying a preconstructed deck is a great place to start - particularly event decks, Planechase decks, and Commander decks. These three types of deck will serve as a fine entry point into the format you're wanting to play. Likewise, there are plenty of budget deck building forums online, and these are a great place to generate ideas. Most budget forums have price tags along with decklists, and will give you a good idea of the amount of money you will have to spend on a deck. Just be sure that the deck you're considering isn't a weaker version of an already existing competitive deck!
VII. Thou shalt have a plan to win. As obvious as this seems, one of the traps that many budget deckbuilders fall into is putting together a deck full of good commons, uncommons, and budget rares without a solid plan to win. A mono-green deck chock full of elves and ramp spells doesn't do anything on its own. Once you acquire all that mana, what do you plan to do with it? How do you plan to get the spells you're ramping up into in your hand? Left to your own devices, how many turns do you need in order to complete your win condition? These are all questions that need to be answered before you take your deck to the local game store.
VIII. Thou shalt NOT over-specialize. This is a trap that tribal deck builders fall into quite often, but also one deck builders are susceptible to when they think they've found a weakness in the meta to exploit. If the current Standard is not prepared for Discard + Graveyard Exile, then it's easy to stuff a deck full of 20 discard spells and exile tech. That's not going to win you any games, however. Eventually you will have obliterated your opponent's hand and graveyard, and your opponent is in topdeck mode. Chances are, you won't get to make him discard anything else the rest of the game. Likewise, you have every chance of running into an aggro deck that empties his hand on his own by turn four, and you suddenly have twenty useless cards in your deck. Even the most specialized decks have room for other cards- goblin decks have burn spells, combo decks have counterspells or protection, and control decks have something for a win condition.
IX. Thou shalt NOT neglect thy sideboard. Under no circumstances are you ever to say, "I'll just save money by not having a sideboard." There is a random deck or two out there that might not benefit from a sideboard, but you're probably not building it. On the whole, cards meant for the sideboard are usually inexpensive, and you need answers to cards that cause your deck problems. For example, if you're playing a mono-colored deck, a single creature with protection from your color can shut down your entire game plan - so you need ways of dealing with those creatures. Additionally, sideboards can give you additional deck building options. Mono black control, for instance, wants more discard and creature destruction options in its sideboard so it can maximize its damage against the particular deck its opponent is wielding.
X. When you have the money, play Limited. Drafting is a fun way to spend an evening or afternoon, and it'll help you build a collection of cards to try in decks, or to trade with other players. Limited formats are an excellent way to learn the game, to practice deck building with limited resources, and discover cards you didn't even know you liked. Prereleases are an excellent time to play Sealed, and there are numerous players at these events looking to trade cards they have for ones from the new set. Unless you're an experienced player, however, seek drafts that aren't Rare Drafts (where all cards go into the prize pool). If you're using the drafts to build up a collection, then you want to keep all the cards you draft. Most drafts at your local game store are drafts that you get to keep all cards you draft. Rare drafts are usually played at someone's house on an invite basis.
These simple rules have served me well in most CCGs, but especially Magic - Commander in particular. If you start with a $50 budget, build your Commander deck, and find you enjoy playing it, it's usually simple to add a card or two every now and then to improve the deck when the opportunity arises. Before too long, you'll find your "budget deck" has become a force to be reckoned with!