Value is too personal for us to really "put a price on it." Like Mises says, people value based on their individual goals and motivations, which is the same way you value an appropriate bounty amount, as well as who deserves that bounty.

I suggest experimenting with the engagement you get, and raising or lowering the bounty next time until it's suitable for you. There's going to be diminishing returns at either end (too high or too low) until you find "the golden road."