That's great, except that's never happened.
For example, digital games also do not have the ability to resell them, but they aren't inherently cheaper to compensate for this. This is because creative works are not priced cost-plus, they are priced by perceived value.
Insamuch as digital textbooks will be cheaper than physical, it will be purely a psychological pricing thing rather than a function of the publisher's expected profit. If that $100 textbook was $90 used, the digital copy will be $85, even though the publisher would still get the same total revenue selling them for $10. They will bank on students not accounting for the used market at the end of the semester lowering their total costs (or students not having extra cashflow to lock up in textbooks).