The math on the valuation of the shares is a little more complicated than 1000mm - 300mm / 24mm.
The rumor that I heard (need to substantiate somewhere) is that the options were priced at a Facebook valuation of around ~75bn. It's impossible to accurately price these options without knowing the total # of Facebook shares issued so far.
Shares come in different classes with different valuations in any case, there's no single number. Presumably zwigby is talking about the eventual market price of a post-IPO public share. But of course there's no requirement that these be issued 1:1 with the existing shares.
