The Appraisal Institute publishes several good books on the appraisal of Real Estate. One of the approaches is the Income approach. There are also some good books on Business Valuation. Quick and dirty, you can take the net income before taxes and divide it by a rate. That is where the expert comes in. What rate? 10! Ok so it is not always 10, but for a quick and dirty 10 works. In reality you have to determine the risk of the income stream continuing. The more the risk the higher the number. As I said, in Real Estate it generally hovers around 10. (I am going to hear from the appraisers out there!) This method treats to property like a bond, ie if you want a 100 a year you need a $1,000 bond at 10%.
The other way to do it is a discounted cash flow. Once more you are getting at the net income and discounting it at some rate. A safe rate, the rate of treasuries would not seem practical, neither would a highly speculative rate if the company is up and running with a good track record. I personaly think this is a better way but involves more spreadsheeting than one is normaly prepaired to do.
So, whats my answer. I would treat the income as just that. How much risk is involved in order for me to get this income? How long will it last? Will there be a residual value? Am I going to have to put in 'Management' time. If so, will I get paid for it, or does it come out of the net? Not sure I gave you the answer you are looking for, but I would start at the Library.
