E-textbook vs print textbook costs

A $14.95 textbook from Apple and its collaborators? That sounds like a heck of deal compared to the average $72 we are currently spending on the dead tree version. But let's play with the numbers a little and figure out an estimated cost for providing textbook access in both formats for a student from grades 7-12.
Paper text:
6 years X 6 courses X 1 textbook per course X $12 ($72/6 years use = $12 per year) = $432 per student
E-text:
6 years X 6 course X 1 textbook X $2.50 ($15/6 years use = $2.50 per year) + 1 iPad @ $500 = $590
There are some major, very likely erroneous or impossible to know, assumptions in this calculation:
- This calculation assumes that e-texts can be purchased once and then re-used each year like paper textbooks. So far, publishers have not applied the first sale doctrine to other e-books, so I wonder if this will be legal.
- This assumes that a single iPad will last six years. The oldest iPads are only two right now, so there is really no way of determining the life-span of the devices. Were I a betting man, I'd give them closer to 3 years of reliable life when used by kids.
- This calculation gives paper textbooks a 6 year lifespan. 10 years of use is more realistic in many districts.
- This calculation does not factor in maintenance, support, and training costs of the iPads or the e-textbooks.
- Might e-textbooks' "value-added" features of videos, interactive tutorials, etc. decrease the need and associated costs of tutoring, special education services, and remedial work?
- Would purchased e-textbooks provide a bridge to more self-written texts by teachers, lowering overall textbook costs?
- What is the value of having a device like an iPad for purposes other than accessing e-textbooks for communications, collaboration, productivity? Decreased printing costs could be factored in.
Yeah, I'm being a Philistine here, just looking at costs without mentioning the la-de-dah transformational possibilities, the evilness of Apple's proprietary formatting, and other philosophical conundrums.
I'd love to hear other ways of calculating the TCO of e-textbooks...