$500 is about how much I paid for my Trijicons (I've got a few of them). Barely used. If you monitor the various gun boards you can find great deals.You use a $1200 scope on a $500 rifle! I am looking at scopes for my next 5.56 AR build and my budget is $500. (About 1/3 of my budget for the whole gun)
Check out AR15.com and Sniper Hide. Good scopes are always for sale there.
Good luck!