nomis5 - 9:28 am on Feb 10, 2013 (gmt 0)
I don't think there are any, not ones that are any use.
There are too many variables such as how much work to maintain the site, is the income steady / falling / rising, how old is the site, development potential etc.
I do know that some sites go for less than one years income and others for more than four years income. It's a very subjective decision as to how much a site is worth.
I would imagine though than any site is now worth less after the last couple of years changes at Google. It's now blindingly obvious to anyone that a site can be OK today and obliterated tomorrow if it depends on traffic from G.
Two or three years ago many believed that a good site would be safe for years - that's gone now.