Depending on the model, probably starting at 1100.00 and up. There are deals out there, but the Colt's do hold a premium.
For curiosity, I went over to AR15.com and checked the EE.
I found pre-ban Colt's as low as $1100.00 for a Sporter, up to 2750.00 for a model 6520 configured as an M4. I can't comment on the % condition of the used, and none mentioned having the original box.