Like other people said, you can buy a 2-4 gallon kritter keeper (link)
at most chain petstores for about $10 or so. They can accommodate a small 25 watt heater as well. Some people have had success using the cheap preset tetra heaters that are only $15 or so in these containers. Although it is agreed that adjustable heaters are better. Also make sure to check out your craigslist! Many people give away used tanks (sometimes with all accessories included) at really good prices! I just saw and ad on my craigslist where someone is selling their 10 gal with decorations, heater, plants, filter, gravel, and light hood all for $50! I was sorely tempted to get it.