depending on the kit you get, you can spend between $800 and $1500 for the kit, plus another $250 for installation, depending on labor rates in your area. vintage air has a kit for your stang. you can get them through most mustang suppliers.
I have the vintage air unit and I'm really happy with it. It was under $1000 for the kit. The vents that come with any of the aftermarket kits are cheap looking IMO. I reworked a set of original vents to work with the kit
Yes it is possible to put A/C in a 66, or pretty much any year for that matter. Check out Classic Auto Air or other vintage A/C suppliers. http://www.classicautoair.com/
I may be considering selling my 1967 Factory in dash A/C setup....
I'm beadblasting/restoring all the pieces now anyways... not that they really needed it.
I just purchased a system from Classic Auto Air. ($895) They have the 7 cyl Sanden compressor (vs the 5 cyl from other companies) and a redesigned condenser that sets their system apart from everyone else. Also you might want to install an electric fan for those long stop lights. I am going a couple of steps further with mine. I also bought a 180 amp polished 1 wire alternator,got the optional polished compressor,all billet pulleys and an Edlebrock Victor high volume polished water pump. I already have a Griffin radiator so it should really shine under the hood when I am finished. Oh yea, this is going in my 65 Coupe. Should be a simple straightfoward install.