The fee is expensive, when you use a BTC atm you should expect to pay fees as high as from 7%-20%, it all depends on the atm operators and how much they charge, these operators would fix their charges or fees based on their operational cost in the business, so if you are not ready to pay fees as high as this, don't use a BTC atm. However, take note that after buying BTC from an atm, if you want to send the coins to another BTC wallet, what you are going to pay is on-chain tx fees to miners to confirm your tx.