Traders incur trading costs to do trades using the Bisq trading protocol.
Trading fees are paid to the Bisq DAO to fund development of the Bisq software.
These fees are payable in BTC or BSQ, with BSQ fees targeted at offering a discount of 50% over BTC fees.
Bitcoin mining fees must be paid to do on-chain bitcoin transactions, and each Bisq trade currently requires 4 on-chain transactions: maker fee tx, taker fee tx, deposit tx, and payout tx.
Offer makers only pay for the first one (maker fee tx) while offer takers pay for the other three (taker fee tx, deposit tx, and payout tx).
Another potential "cost" is the difference between spot price offer price (also known as spread). Spread is set by offer-makers and tends to go down as liquidity improves.