[drc500free]

Under GAAP rules, you could use FIFO, LIFO, or average for inventory costs. IIRC firms generally use LIFO, since that usually results in higher cost of goods sold, and therefore lower taxes. They can't do exactly "most expensive first," but LIFO is pretty close to that since inventory prices tend to increase.

Tracking how many units were bought at each time at each price is not part of the core accounting ledgers of debits and credits, that would be supplemental info that helps you determine how large the debits and credits should be whenever you use up inventory.