Often, I'm asked, "Aren't all businesses worth the same multiple of profit or percentage of sales? Profit is profit, and a dollar is a dollar, right?"
To answer, I usually ask an absurd question: "Which is more valuable, the business that requires getting out of bed at 2 a.m. and driving through alleys of San Francisco collecting recyclable trash, that grosses $500,000, and nets $150,000; or, the business that rents out chairs and umbrellas on the beach in Santa Cruz on warm, sunny days, and also grosses $500,000 and nets $150,000?" Of course, the obvious answer is that the beach chair business is far more valuable. It's all about commitments of time and the lifestyle enjoyed by the owner.
And, that's what "rules of thumb" answer. How is this particular business or industry regarded by previous buyers? How did they value the gross and net revenue because of the type of business? With the right data, we can calculate the numbers and find the ROI, EBIT, EBITDA, "net cash flow", and "SDE"; but, only actual past human behavior indicates what those abstract numbers mean to real buyers.
Business brokers are very familiar with two typical ways of providing a quick value estimate for a small business, using comparable sales of similar businesses: (1) a percentage of gross revenues, and (2) a multiple of seller's discretionary earnings ("SDE"). These work fairly well at estimating a realistic fair market value range in the majority of cases, especially when the business is not all that unique and there are plenty of comps.
But, in those situations where the fair market value is more complex because the business is unusual or nonconforming and there are few comps, I use a variety of methods. One is to calculate the full replacement cost of the physical assets (adding 25% to cover sales tax, acquisition, freight, and installation costs), plus the cost to replicate the customer base, plus any cost to replicate intellectual property, plus a value on the time-to-market advantage of buying an existing company rather than starting from scratch. But, there are many other methods, with the ultimate purpose to determine the potential actual value of the business to the most likely buyer.