Aircraft that appear to be equal mechanically may be very different financially when on lease, the terms of different ones of the same type can be surprisingly far apart, to the extent that the airline may look to get rid of one in short order to replace it with a comparable one on much better terms, if they can't "renegotiate". Then some may be on lease, others outright purchased - but later sold to a lessor and leased back. I think that Fred Newman at Dan-Air liked to buy older frames "for cash", and run them to end of life. Their newer ones in the fleet were nevertheless leased. Airframe hours, next D-check, etc, and the overall basis on it being hours-based, monthly-based, etc, all plays a part in the costing, and in the negotiations standoff when taking aircraft on.