2000 Ford Explorer themostat

My themostate needle barely gets above the "C" line and then goes back down. Coolant in the radiator is full, as is the overflow container (up to the cold full line). I'm good for temps -35 to -40. I get warm air from the vents but never gets hot. Could there be something loose or do I need to replace the themostat? If so what is the average cost for a garage to do this repair and or the average cost of just the themostat.
Thank you for your time.
Judy