Go here :
http://www.p38forum.com/. List all markings on the weapon and magazine and these guys will tell you when it was made, by who, etc. As JK said, more P-38's were plated by GI's than you can imagine. They weren't thinking about rare collector variants or what they would be worth 60 years down the road. Rough value estimate would be in the 300-500 range.