I think it is more like [World GDP in US$] x [% of BTC acceptance] / 21 Million

World GDP = $52.9 Trillion [1]  (2009)

So lets say BTC takes over 0.01% of the world economy

$52.9T * .0001 = $5.29B / 21M = $251.90/BTC

[1] - http://www.google.com/publicdata?ds=wb-wdi&met_y=ny_gdp_mktp_cd&tdim=true&dl=en&hl=en&q=world+gross+domestic+product