We had it done for $2500 for a 4 ton compressor. We needed nothing but the compressor, associated plumbing to run to the forced air unit in the garage, power line hooked up (box was already there) and a new thermostat. The house was already plumbed for central air when it was built in 1994.
Our neighbor knew someone.
BTW-If you have a forced air heating system your house may be plumbed for central air as well. Look for an electrical box somewhere on the outside of your house with a preinstalled drain nearby. If you have that then all you'll need is the stuff I mentioned. We got quotes in the $4-5000 range and up for higher level name brand systems. I forget what ours is but the compressor is the same one that Carrier uses. It's a 13 SEER IIRC. We've had it for about 3 years now.