You may not want a cheap fouton, depending on what you plan on using it for.
If you plan to sleep on it, or plan on someone sleeping on it more than once in a while, buy a nicer one. The cheap ones are uncomfortable as hell, and the mattresses are thin, so you feel the bars underneath. A cheap fouton feels like a cheap pull out bed inside a couch... Awful.
Also, if you plan on using it heavily as a couch (like your main couch), then I'd get a better one, since the cheap ones are not durable at all.
If its for light use and not for sleeping on then a cheap one from Walmart should be good enough.
My bed is a fouton and its great. Its one of the nicer ones but I got a deal on it, I paid about $300. Very sturdy, very comfortable, converts easily to and from the couch position.