"Where do you take the number $500k from? Just imagination, some sort of calculation?"
The world gold supply is valued at around $12T. Should bitcoin effectively replace gold as the global decentralized ledger of choice, each BTC would be worth ~$550K in 2013 dollars ($15T/21M).
Even if that sounds crazy, but the number would be even higher.
I am very sure that at least 1 mio Bitcoins are lost forever.
The real question is, will Bitcoin ever be used like gold/gain gold's market cap?
The velocity of Bitcoin in such a situation would be much higher then the velocity of gold. Bitcoin is much easier to send around the world. Even with significant lost Bitcoin, it could fill gold's role with a much lower price than half a million each.