Currency conversion is the cost most often missed. When the account, the deposit method and the bank use different currencies, money can be converted twice on the way in and twice on the way out, each time at a rate that includes a margin. Holding the casino account in the same currency as the bank account removes most of it. Where that is impossible, the conversion rate on the payment page can be compared with the rate the bank offers.
Payment methods add costs of their own on top of anything the casino charges. Wallet providers often take a percentage when money is loaded or withdrawn to a bank, card issuers may book a casino payment as a cash advance, and crypto transfers pay a network fee set by traffic at the moment of sending. None of these appear in the casino's terms, so the method's own price list is the only place to see them before paying.
Operator fees usually attach to withdrawals. Some sites charge a flat amount for each payout, others a percentage, and many waive the fee for one or two withdrawals a month and charge for the rest. A few charge for withdrawing a deposit that was never played through, which is aimed at people using the casino to move money. The fee table in the payment terms lists each case, and it changes more often than anything else in them.