You might be a bit out of date here. Textbooks are absurdly expensive these days relative to computers; Stewart's "Calculus" is $289[0], for example. That's about the right range for a new low end laptop or a used mid-range laptop.
Perhaps. But 1) you can rent that book for a fraction of the price (Amazon currently shows it for ~$60) and 2) just because students have to pay for expensive books doesn't mean they have extra money to spend on a second computer.