Price per wafer compares the invoice, not the process. What a buyer actually pays for is the number of wafers that survive thinning, bonding and lithography — and that is set by TTV, bow and surface damage long before the invoice is written.
A batch with tight TTV can be thinned to target in one pass. A batch with loose TTV either needs a slower process, or loses the thin end of the distribution to breakage and rework.
This is why we report measurements per wafer rather than per batch, and why the data sheet travels with the shipment: the buyer should be able to see the distribution they are planning around, not an average that hides it.
Where a drawing calls for a number we do not routinely hold, we say so at enquiry stage rather than after the batch is cut.
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