That leaves some (but not all) up to chance. Is someone deserving of a prize if they happen to stumble into a chip that clocks well above what other samples are capable of?You should have a contest of whoever pays you the most for it, wins.
But if this actually seriously goes somewhere, then I agree with Zero about overclocking. Maybe SuperPi scores, 3d mark06 scores, etc. could be used to judge the winner or just the gain % in clock frequency.