HVAC question...whats estimated cost of new air conditioning coil installed on a carrier A/C home unit?

In the range of $750.00 for a 3-4 ton unit. At less than 5 years there may still be a manufacturer's warranty in force. Be sure to ask.

When I change out the coil, if it is electric heat or heat pump, I always look at the blower wheel for fungal contamination. It's easy enough to replace it at the same time. Likewise, with a gas unit, I look at the plenum(the ductwork above the case coil) for mold contamination. A new 4-piece plenum is no big deal, but it could make a big difference to your family's health.

Make absolutely certain the condensate drain line is thoroughly cleaned before the tech leaves. Your new coil is going to condense a lot of moisture, and unless that drain line is clean, you may experience a minor flood. A good mechanic will wait until he sees water draining from the end of the line before leaving.

$750 is right in the middle of the road, good answer.

just did one of these this week. The home owner said it was about 4 years old and that someone had to add freon every summer since it was installed. He had this documented, and we got him a new coil because his was defective from the factory.

Always ask about warranty.