Valuation is hard, especially for startups. People view Facebook, for example, as an "option" on whatever smart business can be built out of 500 million address books and the world's largest photo-sharing service. FB can tax companies like Zynga in order to extract most of the economic profit those folks make.
In this case, though, it's a lot simpler.
(Value of shares purchased) / (Percentage of shares purchased) = Market value of 100% of shares.
It's the same way you'd calculate the market value of, e.g., Google. Share price times shares outstanding. In Google's case, the market is more 'efficient' in the sense that it's easier to buy and sell, and there's more information. But private company stocks offer a different kind of efficiency: the amount of research, compared to the size of transactions, is far higher. Ask a typical economist, and that's inefficiency; but ask Warren Buffett, and it's not.
In this case, though, it's a lot simpler.
It's the same way you'd calculate the market value of, e.g., Google. Share price times shares outstanding. In Google's case, the market is more 'efficient' in the sense that it's easier to buy and sell, and there's more information. But private company stocks offer a different kind of efficiency: the amount of research, compared to the size of transactions, is far higher. Ask a typical economist, and that's inefficiency; but ask Warren Buffett, and it's not.