Most commercial real estate investors will use a cap rate valuation. This is the net income divided by a cap rate percentage that is established by the local and national market trends. The lower the cap rate you use the higher the valuation. Prior to the economy bust a few years ago self storage was getting very low cap rate valuations, I am not sure what they are getting now because I am not involved in the industry any more. Here are some examples:
Gross Income $100,000 less expenses $40,000 leaves $60,000 in net income.
Valuation at various cap rates:
60,000 at 10% cap rate = $600,000 valuation
60,000 at 9% cap rate = $666,666 valuation
60,000 at 8% cap rate = $750,000 valuation
You can look at recent sales in you area and back into what type of cap rates they are using in your area. Your bank might also give you some idea on the cap rate to use (although banks are usually very conservative in this area).