if you are set firmly in the $500.00 price range, and are looking at new handguns i would suggest the following:
CZ 75B- Around $550.00 here
Beretta 92-Around $550.00 here
Ruger P series-Around $400.00-$450.00 range
all are DA/SA, all can be had in 9mm, and all but the Ruger have steel or aluminum frames.
If you look for used handguns, then you may be able to find some Sigs in that price range as well as some lowered prices on several others that have been mentioned already.