In comparing the cost drivers of physical vs electonic books, in addition to physical manufacturing the cost of physical distribution must also be considered. The latter includes both labor and fuel costs.

I would have imagined these physical costs to be significant, though not the main or dominant cost drivers.

I would also love to see some numbers.

Update: Clearly ebook distribution has costs that physical book distribution does not. The implication here is that I believe the marginal costs of electronic distribution, on a per-book or even per-title basis, to be minimal relative to physical. But I acknowledge that this too is only in my imagination and not based on empirical fact.