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-ratesI mean if the price is changing like for now bitcoin is 7k2$ so where is the source of odds exchanging to other currencies? Sorry for my bad english but i guess you can barely understand me.
We don't know where there source it but just like the different trading platforms, they have their own way of adjusting the rates. I don't know if there is an online casino that depends on preev rates.