Rubicon order books are on-chain, all transactions require network fees (gas) that are paid in ETH. Gas fees on Optimism are 50-100x cheaper than Ethereum Mainnet, you can monitor them on this public dashboard.
Paid by Swaps, Market Orders, and orders that 'take' liquidity from the order book
Maker Fee: 0%
Limit Orders that 'make' liquidity on the order book do not pay fees other than gas
Liquidity Pool Fees
Withdrawal Fee: 0.03%
A withdrawal fee is paid back into the pool, effectively rewarding remaining liquidity providers. Stops pool spam.
(These are variable protocol parameters and could change)