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.
Like I have said before it depends on the current buy order in trading platform used by the payment gateway. For example if gambling site is accepting bitcoin with bitpay than price will be the price of bitcoin in the trading platform where bitpay will immediately exchange the same amount of bitcoin so that they can remain safe from price volatility while providing best exchange rates for merchants and users.