I think an 18" XDR-type screen with Pro Motion and the same proportional specs would be very expensive.
It would most likely be cut from the same motherglass as the 16 and 14" screen which should make it about 30% more expensive. But that isn't what was discussed and there is no question that such a MacBook would come at a hefty premium for the user to pay.
The discussion was about how expensive the development would be and I stand by my point unless the go crazy with the specs that it wouldn't be prohibitively bad.