Yes, drop rates and circulating are different things. But the buyer always has the choice - buy from the secondary market or buy a pack?
We know that a pack costs 2 USD and has about has a certain percentage chance of common/rare/epic/legendary. So there is an expected value to buying a random pack.
Right now the numbers are too small and create big variances. When the game is in full swing though, that choice between buying a card and buying a new pack will be the fundamental pricing mechanism.