This has come up often in the past at various blogs, siteflipper sites, Flippa and elsewhere. Some have recommended that one way of valuing a site is to value it based on the free traffic it gets. That traffic is usually from SEs but can be from other referrers as well.
The theory is that you estimate the cost of that traffic as if you were paying for it and value the site based on how much that traffic would have cost if it were bought. The calculation is [Traffic Volume * Cost of Keywords = Traffic Value]. Some even advise multiplying that by a fraction to decide how much to pay for the site and even suggest very exact figures such as 0.45 i.e. [Traffic Value * 0.45 = Fair Price]
When buying a site would you be willing to pay based on "Traffic Value"? If yes, can there be a standard multiplier, what fraction would you use?