Have they provided you with a fleet utilization report for the time the car was being fixed? Most credit card insurance will require them to submit this so they can tell if they were sold-out or not during the period the car was out of service. Otherwise, it wasn't really costing them anything from "loss of use" because it would have just been another car sitting on their lot.

Back to the original "diminished value": Can you search the internet to see what really is the "generally accepted" calculation? And also require Avis to show you where in the contract that it states they can charge this to you?