If the assets have been depreciated over the years to a level of zero, there isn't anything on which to base a book value. The most common means of judging any business is by its return on investment, or the amount of money the buyer will realize from the business in profit after debt service and taxes. However, don't confuse return on investment with profit. They are not the same thing. Return on investment is the amount of money the buyer puts into the business measured against the performance of the business.
Profit is a yardstick by which the performance of the business is measured.
Typically, a small business should return anywhere between 15 and 30 percent on investment. This is the average net in after-tax dollars. Depreciation, which is a device of tax planning and cash flow, should not be counted in the net because it should be set aside to replace equipment. Well, there's a reason for a depreciation schedule. Eventually equipment does wear out and must be replaced, and it sometimes has to be replaced much sooner than you expect.