There are two separate questions here, and I'm not sure which you are asking.
Why should the total value of all Bitcoins be $5.1 billion? (Is the Bitcoin economy overvalued or undervalued as a whole?)
Why should the number of arbitrary divisions of Bitcoin be set where it is, giving each a value of $385? (Does such a high value make it more awkward to talk about and use in everyday transactions?)
the price of bitcoin is decided by the market, the balance between supply and demand! No matter how high the price is, if demanding of bitcoin is huge, and the high price like $500 or $1000 is recognized by the masses. Then the price is stable and seen as true value.