Watch the px or put out a WTB add for a Burris XTRII 3-15 in the matching mil/mil or MOA/MOA, whichever you prefer. Proven optic and that 3-15 is the sweet spot for optic clarity in the XTRII line. I paid $650 for mine
Sightron SIII is another recommendation in that price range in the used market
I know it’s a bit above your $500 mark but worth the extra for a decent optic in the lower price range