Nothing. There are no withdrawal fees from Swan. It is absolutely free!
Currently, our partner, Prime Trust, pays all of the miners' fees required to get a withdrawal of Bitcoin from Swan confirmed to the Bitcoin blockchain.
Bitcoin miners' fees are small amounts of Bitcoin paid to whichever miner that includes the transaction in the next block. They are used to incentivize Bitcoin miners to confirm transactions.
Most other exchanges do charge withdrawal fees. This is just another advantage you have in saving money, and stacking more satoshis at Swan Bitcoin!