Some casinos charge a small fee for processing withdrawals. The same casinos are also likely to either have a surcharge on deposits and certain banking methods.
The withdrawals can vary from casino to another and can also be applied on specific payment methods. Make sure that you read the terms and conditions of any casino you are thinking of signing up to. This ensures you're aware of any potential fees you might incur.
Online casino banking fees are getting rarer. Although some operators still apply surcharges, there are many more that don't.