I haven't used these guys before but they are running a good deal for police trade ins Glock 21 with nights sights and 3 mags for $399.99.
http://kingsfirearmsandmore.com/product.police-trade-in-glock-21sf-45acp-131-3-magazines-13-1824
http://kingsfirearmsandmore.com/product.police-trade-in-glock-21sf-45acp-131-3-magazines-13-1824