Lets assume you're talking about a high school, they have four grades, 9, 10, 11 & 12 and are on a semester system. How many classes does each student take a semester? 10? 7? (My high school was run like a college so my experience isn't typical. We took 10 classes each semester and had 2.5 textbooks per class (english classes often had 6-7 books) Each class requires how many text books? 1.2? 2?
If we assume lower numbers for a typical high school, 4 grades, 1.2 text books per class, 2 semesters per year, 7 classes per semester and a textbook cost of $80 is 4 x 1.2 x 2 x 7 x 80 = $5,376 in text books to put one student thru the whole four years.
Alternatively, the school could buy one iPad, and with an average textbook cost of $15 and everything else the same the calculation is: 4 x 1.2 x 2 x 7 x 15 + $500 = $1,508.
We can assume in both cases, the iPad and the Texbooks last 3 years (6 semesters, and 6 different students) before having to be replaced.
To me, the iPad solution looks cheaper, and all the other benefits- less weight, more interactivity, etc, come for free.
Edit: Left out a term in the second calculation, so corrected, and using asterisks for multiplication resulted in bad formatting, so fixed.