In order to calculate cost basis, we look at the fair market value of the amount being paid (left side of transaction) at the moment of the transaction. So for example, if you are paying $1,000 USD for 1 BTC, then the cost basis is $1,000. If you are paying 1 BTC for 10 ETH, then your cost basis is the fair market value of 1 BTC at that time. For fiat currency, it's easy — the value of the fiat is the cost basis. If it's a cryptocurrency payment, then we look up the fair market value of that coin at that time.
All calculations default to a first-in first-out (FIFO) model. We also support additional methods of specific identification (such as highest-in first-out [HIFO] and last-in first-out [LIFO] from the settings). Read more about capital gains methods in the Tax section of the Help Center.