Offer him $275, and if he says that's too low, tell him you have to deduct the cost of the parts needed to fix the gun.
Here is what you'll need (from Brownells, my favorite supplier of stuff like this).
New style thumbpiece:
Thumbpiece nut (required for mounting):
Shipping should be less than $11 (unless they've changed it recently; possible), and these parts might be light enough to get the small parts shipping rate, which will drop the cost even more.
Good deal on the 686. I'd have jumped on that so fast there would have been thunder when the air rushed-in to fill the space where I had been standing...