...... You can find it pretty easily by seeing what rate they give you for exchanging 1 ETH to BTC (or any other pair) and comparing it to the spot price on CMC or a major exchange. You'll find that Bitsler's rate is 5% lower and Crypto.Games is 3% lower.
I visited gambling websites mentioned in the OP and calculated their fee based on ETH/BTC rate (except for Coinpot.co, for which I used LTC/BTC rate.)
AmoreJaz, you can add the fee rates to OP from the table below.