if you just need heat, a brew belt and a temp controller will do the trick nicely. if you're handy at all you can grab one of those STC-1000 temp controllers from ebay or amazon and wire one up for around $30-40, which is way cheaper than buying a pre-made controller. a brew belt can be had for ~$20.
cooling, on the other hand, tends to be more expensive. converted fridges and freezers are the most popular option (you can use that same temperature controller from above), but those tend to be pricey unless you can score one cheap on craigslist or happen to have one lying around. I've seen other people build a fermentation chamber with 2 compartments - they put ice into one and hook up a fan to blow cold air into the other when the temperature gets too high. that's cheaper than converting a freezer, but you have to monitor it and keep it stocked with ice so that it doesn't get too warm. I've also seen people rip the cooling element out of an old dorm fridge and use that in their fermentation chamber, which could be a happy medium between the two (especially because most people have a mini-fridge around somewhere).
check out the DIY forum here for tons of examples of all these things.