Banking fees are dependent on the online casino and the money transfer method used. To give you some idea, they can range from no fees at all to 5% of the total sum being transferred.
Many casinos don't charge any fees at all; some have a flat fee for every transaction; others apply different sums and percentages for each banking method.
In order to avoid nasty surprises, you should always check the banking fees before making a deposit. You usually find this info on the casino's banking page or FAQ section. You may need to log in to see the exact fees for each banking method.
Although some banking methods may be free to use in the casino, they may not be eligible casino bonuses.