The easiest way to do this is to seek the advice of a business valuation expert. That person’s advice will be impartial and based on experience and training.
Either way, you’re going to need facts and figures to support the sales amount. You can list all the businesses’ assets, both tangible and intangible, and put a value on them. You can review accounts, weighing future profits against costs, and put a value on them.
Unless you’re going to sell your business to family or an employee, you’ll hire a broker. A broker can also give you advice about determining the sales value. The broker may look at similar sales within the industry.