Donal, perhaps you could clarify the question? I read it to mean that you want to project the value and income of a adsense driven website as compared to an investment (return and value).
To paraphrase, would I make more money from the website than an investment over a 10 year stretch for example?
I dont think that is a good comparison if that is what you mean. You might be able to project earnings (very roughly) if you have collected data from your website over a reasonable amount of time probably years. Make a few projections ranging from stable to moderate growth assuming you are going to improve the website. Build in the uncertainity of traffic, conversion rates and the adsense program and your projection will be quite shaky.
My question was a bit long winded. I wanted to know how you'd value a domain based on it's current AdSense earnings.
I also asked a risk assessor I know. He knows nothing about Google Rankings or AdSense, but he wanted to know to quantify a) the probability my site losing rankings (thus AdSense revenue) and b) the likelihood that AdSense revenue will increase.
Reminds me of stocks and their disclaimer. Use historical data to project into the future and use a disclaimer that past performance is no guarantee of future performance.
Rankings are not fixed and will depend on competition, website modifications and a bunch of other factors known only to Google. You can offset the risk somewhat by maintaining a good link profile and clean website (TOS).
Revenue will also depend on competition, traffic, conversion rate, demand or interest level in product and so on.
Make it clear that though you can influence some of the factors, just like stocks, the investment is at risk.