No heat!!!

I've gotta 1992 G20, The air conditioning works fine but the heater blows out cool air..........would this be my thermostat??? If so how much would a new thermostat cost and is it a bitch to replace?

does the car get hot? have you checked to see if the control cable is moving the heater flap on the heater box? you can also check that all the heater pipes are getting hot under the bonnet. touch each one in turn to test. the termostat is not hard to do and location is easy to get to