Hmm, I was just quoted $3500 for a 3.5 ton system, installed with condenser, pad, refrigerant charge, B-coil and labor. The local utility will give a rebate if the SEER number is high enough and you install a system size they recommend. Might be something similar in your area?
If it's replacing an existing A/C, then the hard work is already done and the price should reflect that (outside AC disconnect and wiring is already in place, holes already through the wall, duct work setup and t-stat wired for cooling, etc.).