There are two normal ways of valuing a business.
One is to take a multiple of the bottom line net profits. Usually somewhere between 5 and 10 times. So if the business makes �100k per annum it's probably worth somewhere between �0.5m and �1m. This is the more usual way of valuing a trading business.
The other way is simply to add up the value of its assets and liabilities. In effect the Net Asset Value as shown by its balance sheet. Therefore value of property, vehicles, plant, stocks, debtor book, bank balances, less any loans, overdrafts, creditors, etc. You may also add an element for goodwill though that's a figure just plucked from the sky really. The Net Assets basis is more likely to be used to value a business in trouble or loss making.
The truth is possibly somewhere in between all of that.