From CryptoKittiesSalesAuction Contract - 0xb1690c08e213a35ed9bab7b318de14420fb57d8c

gistfile1.txt

function _computeCut(uint256 _price) internal view returns (uint256) {
    // NOTE: We don't use SafeMath (or similar) in this function because
    //  all of our entry functions carefully cap the maximum values for
    //  currency (at 128-bits), and ownerCut <= 10000 (see the require()
    //  statement in the ClockAuction constructor). The result of this
    //  function is always guaranteed to be <= _price.
    return _price * ownerCut / 10000;
}