Bitmart withdrawal fees are fixed. This means that no matter how much you want to withdraw, the same fee will be charged. For example, the USDT withdrawal fee is 2 USDT, it is 2 USDT if you withdraw 20 USDT, it is still 2 USDT if you withdraw 5000 USDT. For further information on how much Bitmart charges for each coin, you could check this page: https://www.bitmart.com/fee/en