EPV = Earnings Per Visitor
From a financial standpoint, I have seen many gauge a site's value by number of pageviews (impressions) and by total revenue. IMO, the best way to rate a site financially, is by number of unique visitors and your EPV.
My reasoning - you can have a sticky site that generates many pageviews per visitor but no revenue, or a site that has very few pageviews, but makes money. So, a site with 1K visitors per day sounds great, but if the site makes less than $10 per day, the EPV could possibly be improved. However, a site with 100 uniques per day, making $10 per day, is much more valuable due to a higher EPV.
Unique visitor = 1 pair of eyeballs = 1 shot at revenue, regardless of how many pageviews.
Take one month's site revenue, less operating costs. Divide that number by the total number of unique visitors, and you have your sites EPV. So, if site xyz.com made $5000.00 for March with 30,000 unique visitors, 5000/30000 = .166 - round that up to .17 for a $.17 EPV.
With this figure, you can safely estimate ROI on marketing, buying traffic or SEO services. You can also track your sites EPV from month to month, and look for patterns and warning signs. Regardless of your traffic fluctuations, hopefully your EPV will remain somewhat constant. (Seasonal sites excluded.)
Knowing your EPV before and after any changes can help you gauge your work. If you change your site layout, navigation, colors, etc. you should watch your EPV to determine if your changes helped or hurt. If you decide to launch a PPC SE campaign - knowing if the new traffic hurt or helped your EPV is golden info.
What is your favorite way to measure a site's financial performance?