Well this is more of a matter of opinion than anything. It really also depends on what purpose you're trying to evaluate it for. If it's for a single one time sell to another business you would calculate the worth of the application based off the potential income it will bring to that buyer. If you're trying to determine the value of an application as far as what it would cost to duplicate it - you would base it off a formula of hours * avg. industry pay rate * 50% (overhead). You might also base the price on the current income it's generating * 4 years.