A straight swap (existing C wire, compatible system) runs $189–$249 installed. Add a C wire adapter or transformer and you’re at $279–$349. Full thermostat wire pull from a far air handler can hit $400+.

Nest, Ecobee, Honeywell T9, and Sensi are all solid choices. Skip the no name Amazon brands they’re almost always incompatible with two stage or variable speed equipment.