If you wanted a round number you could round the post proceeds up to the nearest 10 maybe XD
Or maybe a totally voluntary entry donation of 1sbd/steem? (totally voluntary for people who want to help spread the love and some especially new steemates may not have built up to that yet)
10-20 is heaps considering it's a showcase :D
Would asking for a voluntary donation work do you think? This would go in the prize pot naturally.
People do it for some of the other games/comps that are running around the place so I'm sure some will. One way to find out ;D
Good point :)