Service businesses tend to start around 2.5x revenue. But like Patchy said, it really depends on the business type. You can google "business valuation x" where x is the type of business we're talking about. In the type of business you're describing, where it's two guys who do some activity, it's usually a multiple of profit, where one partner will no longer be part of the business. If there are business assets - like lawnmowers for a lawn mowing business - also add in the fair market value of the business assets.
Some folks also provide a percentage of profits for some period of time. This lets the exiting partner benefit from the sweat equity already invested without seeing things through. It can lead to problems if there is not an accepted method for calculating profits - for example, if you take lots of three martini lunches on the corporate dime.