On the Ethereum network, all transactions require a miner's fee, which is normally referred to as "gas". The size of this gas fee is directly proportional to the amount of computational effort required to execute an operation. The more complex the operation is, the more gas it requires to execute. Therefore, it is impossible to calculate the exact size of gas before the transaction has been executed.
As the Ethereum network and its market value grows in size, so do the gas fees required for transactions. At times, these fees can very high and significantly impact the profitability of trades/swaps. You can view current Ethereum gas prices here.