Often this question crops up in business discussion about how to value a business.
Every industry has its own way of valuing. Actual valuation is ultimately a matter of negotiation but some bench mark is needed as a guide post for both parties to start negotiations.
Like in our own accounting business : value is usually 110% of the annual revenues.
I came across a gem during my recent studies, have a look via link below:
This gives a basic rule of thumb used in valuing different businesses.