If we take out all the awesome gift knives I've received, I'd have to say the best deal I ever got on a new knife was a Cold Steel Meduil plain edge clip point Voyager with a Japanese made AUS8 blade for $5.00 at one of Cold Steel's parking lot sales. It's a Cold Steel, but for $5.00 it's a hell of a knife.
On a used knife, I'd have to say that the best deal was the hundred bucks I paid for a limited edition Green Jigged bone handled Joe Kious collaboration Shrade slip joint. I always wanted a Kious of some sort and never thought I'd even get a collaboration knife so this one is special. There was more to the deal as the young maker I got it from was supposed to come visit my shop for a little quality time with my Bridgeport, but he's never followed up on that part of the deal.
The Microtech Medallion for $60.00 was a pretty sweet deal too.