There are two different fees associated with trading:
Trading fees (that are paid to liquidity providers).
As the Bancor network operates on blockchains, all transactions will incur gas costs, which are unrelated to Bancor, but rather the network itself. Different wallets such as MetaMask will show you gas estimates.
Besides the network gas fees, each pool is set with a percentage fee on each trade, which is sent to liquidity providers in the pool.