I looked through about a dozen ar15's at the local gun shops today.  The only one that caught my eye was a 20" barrel, with A2 flash hider and lug, A2 upper, with a factory colt scope on top of the carry handle.  It had long cylinder style (not stock, no rails)  handguards.  A2 buttstock.  It had an olympic lower.  The guy behind the counter told me the scope alone was worth 200-300 bucks, and that the AR itself was only worth about 600.  Is this guy feeding me full of bs?  The scope does say colt on it, and it does not have very much magnification, and you can cowitness the iron sights with the scope on.  I was thinking after I sleep on it, shop is closed tommarow, I would try to barter with him, or get some extra mags, and or ammunition at that price, or if scope is POS try to buy it without for less than 600 like the guy was saying.  I tried to find a similar scope on gunbroker but could not find one.  It is only like 3 or 4 inches long and sits on top of carry handle.

Try E-Bay they sell Colt Scopes.
E-Bay Colt Scope