Assuming that the minimum prizes do grow linearly, we have this ramp-up for the smaller prizes (but that wasn't the main reason the competition was +ev):

7 day: 0.5 mBTC
15 day: 0.75 mBTC
25 day: 1 mBTC
50 day: 1.25 mBTC

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Maybe it'll go up to 1.5, who knows.