A thermostat going bad is generally very inexpensive to fix, and oftentimes you can fix it yourself quite easily (depending on your car). To find the thermostat, it's usually inside the housing on the top of the engine that connects the top of the radiator to the top of the engine. Basically, open your hood, find the thick hose at the top of the radiator that goes towards the engine, follow that hose to where it meets the engine, there should be a metal housing bolted to the engine at that point, and inside that housing is the thermostat. It's usually held on by just 2 or 3 bolts. All you'd do is remove the bolts (have a pan ready to catch the coolant!), remove the old thermostat, pop the new one in, put the bolts back in, and top up the radiator. 5-minute job, tops.
You should be able to do it for yourself for just the price of the thermostat (they're usually under $10 each). But if you'd rather have a mechanic do it, a local mechanic should be able to do the job for you very quickly... definately under $50 installed.