There are clearly many methods and factors determining the value of a website. Here's my two cents....
Made up example;- "blue widgets"
- let's assume bluewidgets_com is number one in google for that exact search term
- to advertise with adwords and be higher than the organic number one may cost £1 per click
- assume 10,000 searches a month for that exact term, that's 120,000 searches per year
- Assuming 45% of those will generally click number one in the serps that's a total of 54,000 clicks, at £1 per click would £54k per year in adwords costs
So, if you are the owner of bluewidgets_com you've already got a rough valuation
before any other site revenue of circa £54k, based on just 12 months worth of clicks.
Of course there are LOADS of other factors, including additional keywords and phrases but I consider this one pretty important.
I'd put this in to a mathematical equation but have been working on VAT today so my Vorderman mode is temporarily unavailable...