With any aircraft, all maintenance required is carried out at intervals set by the manufacturer. All these maintenance tasks have a known expected time to complete, and the cost of components required to be replaced are also known. There fore it is relatively easy to forecast what these fixed costs are.
Of course, nothing ever runs to plan, so a bit extra cost factor is needed to be added in to allow for the unexpected. Of course, as aircraft age, they will require more inspection, and parts will start to break more often.
Try contacting an airline, and see if they can help you with more information, with out giving away any confidential information.