There is no real "fair" - only what you actually pay for expenses. I would recommend going over your expenses for last year (and what expenses you've had for this year thus far), anticipate a slight increase for the coming year and you have your CAM expenses.
Now, the CAM expenses are only one "N" of what is known as "Triple Net" (NNN); with NNN leases, there is also property taxes and insurance. However, in this market, and in the Jupiter/Tequesta area, many landlords are doing "Gross" leases and are paying their own CAM, insurance, and taxes.
Hope this helps.
Jun 15, 2010