I don't think sensor surface is that expensive anymore.
We are not in the 1980's anymore
You seem to repeat this "big sensors are getting a lot cheaper" as a mantra, even after apparently dropping your former confident prediction that steppers would evolve towards being able to make these larger sensors without the on-wafer stitching now needed to get beyond about 33x26mm. (Aside: Canon has discontinued its one larger format 0.5micron stepper, in favor of a new model
fpa_5510iv with far larger 1.5 micron minimum feature size, even more obviously useless for making MF camera sensors. So more than ever, 33x26mm is the maximum field size that sensor makers must work with.)
Do you have any concrete evidence about pricing of bigger sensors, particularly for the larger sizes like 56x42mm and 56x56mm that come up in these "full frame medium format square sensor" discussions? Bear in mind that
1) There tends to be a factor of three increase from the factory door price of a component to its contribution to the retail price; probably more for low volume items that require higher percentage unit price markups to be profitable. So a sensor that sells for $3000 in orders of 1000 probably adds about $9,000 to the price of the back it goes into.
2) The price that can be charged for a very old model with R&D defrayed [like the old 22MP 48x36mm Teledyne-Dalsa-Phillips
FTF4052C] is far lower than the price that must be charged for a newly-developed low-volume sensor.
We are not in the 1980's anymore
No, and one thing that has changed is the continuing patterns of cost reduction through size reduction, with ever
worse economies of scale for the outliers of huge IC's, like camera sensors bigger than 33x26mm.