Ethereum is already something that is cheap and very fast, I do not think that you could really spend too much to actually withdraw.

Nowadays if you spend something like 20 cents per withdraw that is more than enough, and if you are not gambling on a price that makes 20 cents very small I am sorry but maybe you are not playing with enough money to actually justify gambling.

Normally the minimum people play with is 10 bucks and definitely a lot more, when you are gambling with at least 10 dollars or more 20 cents will be very cheap and if you are playing under 10 dollars you are going to get 20 cents felt but it really doesn't justify gambling, that is more like faucet gambling. This is why I think a fixed fee would be fine as long as it is decent.