I'd recommend going to gunsamerica.com and browsing.
Depending on the type of shooting you want to do, one of the best optics for the money I've found is the Burris AR-332. It's got a ballistic reticle that's designed for the 5.56 trajectory out to 500 yards, has a dual illuminating reticle in either red or green, and still shows up black when you're battery dies.
Thanks for the info!