How much does it cost to replace a transformer on the heating and air conditioner?

Depending upon the unit but todays prices are high. Probably 50.00 for the transformer and with service call and labor another 150.00. I would think 200.00 might be todays number.

Mouser Electronics lists a Stancor P/N SWC-824 for $24.42 that puts out 4 amps at 24 volts. Likely you can find one for less than $50 at a hardware or home center. Just make sure the new one has as high, or higher rating than the old. A couple of screws and 4 wires to instal it. If the old one didn't have a fuse protecting it, I would add an inline one for a few bucks more.

No sense paying over $100 for labor for a simple little job. Such charges aren't a rip off, just the huge overhead in service work.