There would be no de minimus limit, so any prize or prize-set could be offered. The ICP creator would need to stump up the cash up-front (which would then be help in trust by the website organistation, with interest accruing to the prize). A variety of rule types would need to be managed, e.g. fixed time limit vs open time limit, best entry wins vs winning only with a defined set of criteria.
If successful, the website in itself would provide significant free marketing to the contest, increasing the number of participants. It would also reduce barriers to entry, resulting in more contests in total.