To be honest, under most other circumstances it would make far more sense to wait until the winner is determined, and then ask the actual winner what they'd like. You can specify the maximum monetary value of the prize beforehand. You can also come up with a small set of choices (eg. steam gift card, or the next humble bundle, or <blah>, ...) beforehand in case the winner doesn't have a strong preference but doesn't want to completely opt out of the prize. With any luck, maybe the actual winner is someone like mobius and then you even save yourself the $15 or whatever the maximum is since the preferred prize would happen to cost $0.
Regarding the current humble bundle situation, if there's a decent chance you think you can resell it in future for equal or higher prize than the $15 or so you paid for, I suppose you can buy it now and hold on to it for some (not too long) period of time, in case someone who has expressed interest in it becomes the winner in the next or future contest.