I bought a cheapish SPOT red dot sight based on a review on Ar15.com and other sites. I liked it so much I bought a second one. So far they have both held up great and the battery life is damn good for a $100 red dot.
Another budget line that most have had good results with is Primary Arms.
I picked up both of my SPOT red dots used and cheap. I will eventually get a quality red dot (Aimpoint) when I am ready to have a heavy duty, SHTF rilfe (the BCM Dissy I'll buy next will more than likely be the one).