I paid $400 for a used HK USP9. There are some excellent deals to be found in the used market. The USP is one of my favorite duty pistols, certainly more than worth the price I paid. Police trade-ins are a good source of high quality, low priced pistols.
Another excellent deal can be found in the Daewoo DP51. This is a very good quality pistol and I've seen them as low as $250. Not as soft recoiling as the HK, but accurate and very reliable. A best buy IMHO.