I think it's impressive that _any_ college expense has come down at all.
Also you're not taking into account the fact that inflation alone should have increased costs by roughly $100 since 2008. But instead there's a net decrease.
Looks to me like they are not calculating the net cost. Since they don't mention selling back and all and then attribute some of the fall in price to rentals.
Most people would come out farther ahead buying on Amazon and then selling back after the semester. Rather than rent.
If their calculations do not include selling the book back, my guess is students are actually spending less up front but spending more overall.
Also you're not taking into account the fact that inflation alone should have increased costs by roughly $100 since 2008. But instead there's a net decrease.