I have an all-black USP9c, Var 1, date code AK, with about 300 rounds through it. I'm thinking about trading it for a SS one of about the same date code [for ex., AG to AK] in similar condition. What would be the price difference between the SS and the all-black used models? This info will be helpful in my deciding to try to trade or to keep my all-black model for the rest of my life. TIA