Good to see the furtherance of the Part 91 RAC stereotype...
I've been 121 for two companies as long as I was a Part 91 Chief Pilot.
As a 121 pilot, I have seen exactly ZERO cost accounting for maintenance expenses - parts or labor. Jet broke, entry went into the logbook, and the company took care of it.
As a 91 pilot, I had no Maintenance Control, no warehouse of parts, no maintainers. I solicited RFPs for and scheduled every scheduled or on-demand maintenance event and saw every invoice every time a technician touched the aircraft.
Yeah, turbine aircraft are REALLY expensive to fly...for airlines, too. Its just as a line crewmember those costs are completely transparent to the act of moving the jet Point A to B.