Actually, I think this was less about cost and more about systemic creep of operational differentiation from earlier versions of the 737. A big selling point for the MBAs was that this was a 737 and pilot recertification was not necessary. So the MCAS system and its deadly potential was hidden from the pilot manual, as was the new failure mode introduced by this system. Acknowledgement that these systems required additional or different contingencies or checklists, or intruding an automatic shutdown of a pressurization would require recertification in type, or potentially even recertification of the aircraft if the changes were significant enough.

Significantly, for MCAS, the reason that the stability of the aircraft had to be patched in software, leading to hundreds of deaths, was that changing the empennage to reestablish aerodynamic stability might have been a big enough change to require recertification of the airframe. That would have been expensive, but it would have also opened the door to fixing all of these other issues that resulted from trying to pretend that the aircraft was not significantly different from earlier versions.

Its bean counters all the way down, and dead passengers is the price of that.