Agreeing with the others...if you are a single op or family op, the value is in you...you can value the equipment in black and white, and you can estimate the potential for business, but the new owner can either drop the ball or run with it with no guarantee implied on potential for new business.
If you are a multi op, you increase your value by selling your network of associates, and if you have advertising and websites or other tools, that also increases the value.
If you can sell the business with a model intact that self propagates income, it would be of more value than a model that requires you to solicit jobs or a model that the income comes word of mouth from your associates.
I am a sole proprietor model. I could sell my business tomorrow for the equipment value, and if I meet the right person, a little more with the guarantee of turnkey jobs...but even if I sell the jobs, who is to say the client wont cancel them when they hear I am no longer involved? If I wait even longer, I may be able to sell someone for more money with a look at my books and earning potential, but when you start looking at that kind of money, it would almost be better off starting from scratch...
So in conclusion, your value to the buyer depends on what, exactly, you are bringing to the table.