Sounds like this is a NNN property with the tenant(s) paying CAM. Hopefully the owners (or managers) have been doing annual CAM reconciliations and if that's the case, then the reconciliation should show the actual expenses for the previous year in a one-page statement.
Just in case you didn't know, NNN leases should have a provision to allow the owner to reconcile the actual CAM expenses versus the estimated one tenants have been paying. If the actual expenses paid that year turn out to be more than what the tenants have been paying, then the tenants have to pay the difference. Vice versa, the landlord should credit each tenant if the estimate is more than actual.
The reconciliation should be very accurate as the lease should also allow for the tenant the right to audit landlord's records including seeing copies of actual invoices paid. I would say that if you can get your hands on past 2-3 years of reconciliaton statements, the statements alone should give you a good idea of actual expenses.