96 GrandAm thermostat

I have been told that to replace my thermostat, a $5 part, the shop would have to take my engine out; and, the labor would be between $600 and $900. Is this true?? Please help me here, I do not have a clue. The heater core was replaced, but the temp. gauge doesn't work right at all. The heat works fine, if I'm standing still. As soon as I start driving, I start freezing my a** off. What the heck could it be? My thermostat? And if so, is the taking out of the engine thing really true? I was also told that it was an easy job by someone else. In case you haven't figured it out already, I'm not too rich. Please help me, SOMEONE...ANYONE, give me some advice.