How to measure the aircraft maintenance cost of a operator?
Maintenance costs are generally expressed as dollars per flight hour for a particular aircraft type.  Chargeable labour hours, parts, overhauls, delay costs, repairs etc., on airplane model X, divided by that airline's total flight hours on that particular type.

This kind of data will be broken down and tracked at the Line Replaceable Unit level as an hourly operating cost of the LRU itself.  Like, say, a hydraulic pump will have all expenses related to maintaining that particular part number, including repairs, overhauls, and chargeable delays/cancellations caused by failures, all added up to a global dollar amount per flight hour for that pump.  This is also tracked by the aircraft manufacturer for the entire fleet. 

That number will be used to assess whether the pump is better or worse than the global fleet for that aircraft type, or compared to the same pump used on other aircraft models, to decide whether to pay for improvements, look into changing the maintenance practices, soft timing the pump (overhauling sooner than the legal requirement), and other actions.  There will be some alarm threshold established, say >$1.00 per hour, or 85 cents/hour or whatever, the airline will use to determine whether something should be done or not.  

If the pump manufacturer introduces a product reliability improvement, it will be sold to the airline as providing X cents reduction in hourly costs of that LRU, along with X percent improvement in dispatch reliability. The airline will do a business case to decide if the long term saving is worth the up front expense.  They usually go by a payback period, where for example the projected hourly saving from doing an upgrade will pay for the cost of the upgrade within 5 years, or less depending on how cheap the operator is (over the last 10-15 years, a lot of US regionals have been only buying upgrades that pay for themselves in 1 year).