The transaction cost is the total fee you have to pay for any swap you make; there are no other hidden fees. It includes our own fee of 0.05%, which is already accounted for in the rate, and the gas fee (gas price), which will be displayed separately.
Gas fees are paid to push your transaction through on the blockchain. If the network is busy, then gas fees will go up. Some blockchains have lower gas fees, while others can eat up half of your transaction amount: the Ethereum network in particular is known for having high gas fees. These days, most crypto wallets, such as MetaMask, set the gas fee automatically.