It's hard to answer your question as everybody looks for something else. Many people go first by cap rate (ration of NOI/price) while others might want to see their bottom number after debt service, or GRM - gross rent multiplier and compare it to similar properties in the area. If you are a novice investor, make sure you take extra time to do your due diligence to go through the leases, financial records, all service contracts, environment surveys, etc.
Another good way of learning about various ways of valuing a property is getting a copy of professional appraisal and reading it from start to finish. Derived value can be based for example on a combination of capitalized income and comparable sales in the area.
Study past sales in your area for comparable properties; translate it to price per door, study rental amounts generated per each unit type, look at their the subject expense vs. gross income ratio and ask other experienced property owners/ brokers if that is typical or somehow skewed to figure out if the NOI claimed is in the ballpark or complete off.
Hope this helps
Aug 4, 2010