You want a ROUGH estimate? About four years ago I had a 3 ton unit
installed (3 tons is the size, not the actual weight of the unit), 16
SEER (that's the energy efficiency, higher # is better) in a 1600 sq
foot 2 story house (basement is not air conditioned) in the NYC area
for under $13,000. (that was the low bidder and they sucked, I
wouldn't make that mistake again- next time I go with a local
contractor, not a chain store's clowns).

That price included having to run all the duct work, and that's the
very labor-intensive part. The units themselves (compressor, air
handler) probably under $5000 plus installation.

But of course you'll have to ask a few contractors for bids, and this
is their busy season for A/C (although it'll get busier).