when you look at what are the expenses of the website and the revenue of it you look at variables.
How many users are getting through the page.
How many servers
Whats the monthly cost per server.
but at the end when poking around you find that there are some nice thumbnail rules.
How many users are making an action that i want them to do in a given time period?
Conversion Rate = Desired Action / Total Number of Visitors
What is the cost of my website per visitor?
Cost per Visitor = Money Spent / Total Number of Visitors
it can also take a look at the number of business leads i produced if they are the revenue makers
Cost per Visitor = Money Spent / Total Number of Leads Produced
but you can get a real informative perspective regarding the users when you look at the value they produce
Value per Visitor = Sales / Total Number of Visitors
and if i elaborate it for a bit the profit you gain is
Profit per visitor = Value per Visitor – Cost per Visitor
this is my gathered information from http://content.websitegear.com/article/conversion_metrics_2.htm