Calculating operating costs for rental properties

[Paul Georgia]

Is there a good rule-of-thumb number for operating expenses for rental properties?  I am going to a tax sale and have identified a few rental properties (2 to 14 units) that I plan to bid on. I've settled on very conservative max bids for each property, but I would also like to check that against a cap rate, but I'm not sure what to use for my operating expenses. Thanks