Polymer- I can't say a bad word about Springfield's XD9, CZ's P-07 Duty, S&W's M&P9. They can all be found pretty close to $450 if you do your homework. That's where my own search would begin if I wanted a sub-$500 plastic 9mm.My question is what is the best 9mm pistol under $450? Preferably full size.
Steel/Metal- There are some excellent "clones" of the Browning Hi-Power (Arcus, FM Argetine, etc) and CZ 75 (EAA Witness, among others) to be found in the $300-500 range. These are decent quality copies of the two guns that set "the standard" for a metal framed 9mm in my opinion.