$50 – $600.
Some cars it’s literally drain the coolant, two bolts, swap out the thermostat and gasket, two bolts in, and fill ‘er back up.
Some it’s a complicated process of first removing a number of other things so you have room to even get to the thermostat (And it’s convoluted housing that is way too close to many other things). And that’s just so you can get started.
The best bet would be to find a mechanic that specializes in the type of car you have and give them a call. They’ll do it faster than anyone else (faster = cheaper) due to familiarity and can probably give you a price over the phone.