I have a Colt AR-15A3 Tactical Carbine marked as a LEO/Government only rifle. It was a Police Only rifle that I got during the ban. It was all of the evil features. Flash Hider, Collapsible Stock, Bayonet Lug and such.

What is the going price for these rifles? I would like to sell it and get me a Sig 556 Carbine.

I don't know about the Evil options, but this may help. depending on condition you can take 1/3rd off the original price.
http://www.impactguns.com/store/COLT-AR6721BF.html

http://www.policeguns.com/shop/default.cfm?CFID=628009&CFTOKEN=80345830&Action=proddetail&ItemID=634&TopCatID=10&SubCatID=106

The price can vary as much as $500.00 depending on the seller/ buyer.