BitMEX does not charge a fee to withdraw Bitcoin. However there is a minimum Bitcoin Network Fee that is paid to the Bitcoin miner who processes your transaction, this fee does not go to BitMEX. This Network Fee is set dynamically based on network conditions.
BitMEX charges a fee on Tether (ERC20) withdrawals. The withdrawal fee of Tether (ERC20) can be viewed on the Withdrawal Page and can change from time to time based on prevailing market conditions.