I think your offer is actually not too bad. You mention worn Coke grips. Depending on how worn they are they may make up for the decrease in value due to guns condition. Perfect Cokes go for $500 to $600 so price accordingly. If the grips are well worn then maybe $200 to $250 for the grips and $250 to $300 for the gun. I'd say the math is on your favor if he takes thate deal.

By the way, if you we're to make an FBI connection which seems unlikely, then you'd get some value back there too.