Most of the fiat based online casino accepts bitcoin using third party payment gateway like bitpay so their conversion of bitcoin into fiat is based on bitpay exchange rate.

You can find current exchange rate here, usually there is 1% deposit fee when depositing through bitpay https://bitpay.com/bitcoin-exchange-rates

Other crypto based gambling sites also have inbuilt crypto to crypto exchange feature and most of them are using shapeshift api for this. Only few have their own exchange rates based on one of the altcoin trading platform they are using for trading.