Roughly, how much does it cost to replace a car's thermostat (by a mechanic)

It depends on how long it will take the guy to do it. Some cars need to be extensively disassembled to do this.

$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.

What kind of car? A Chevy with an accessible coolant line into a V8 will be $40–50. OTOH a V-6 minivan is a nightmare – $200–300. If you give us the specs maybe someone here with access to AllData can get you a price.

Agreed.

We need more information.

This one is older. A 98 Chev Lumina.

$22.86 if you do it yourself, about $190.00 if you have it done at a dealer’s shop. (At least, that’s the estimate range for this part of Connecticut.)

Check out AutoMD.com to check your own estimates.