Hmm, If you're looking for a competitive/tournament deck Mono-Red Burn or RDW work extremely effectively and cheap. Infect is also quite cool, cheap, and modern legal.
Although if you have someone else to play with you regularly(friend, spouse, etc.) then buying a few cheap decks that are balanced around playing against each other is a really good investment to make. The ones posted by themightyjang are an excellent example of this.