Under ordinary circumstances, transactions on xDai are stable at 1GWei, and there are 1,000,000,000 GWei per xDai. To calculate price per transaction, we multiply the gas cost by 1,000,000,000. This is easily converted to USD, as 1 xDai = 1 USD. Since xDai is a stable chain, transactions costs do not vary based on external currency fluctuations.