The sole purpose of book.ly is to help you find the cheapest textbook possible. Search options cover both rental/purchase, paper/digital, and you can track down your required textbook by school/dept/class. It works rather well, and I can't see that book.ly are padding the price so they get a commission. They even quote prices with shipping included.
Book.ly is good, but back to the main topic.
One of the arguments made for digital textbooks is that they sell for less than retail so students save money. This is true, but it is not the whole story. Go use book.ly for a while and you'll see that for most titles, the paper price is cheaper. And that's the buying price, not rent. (Oh, and if you do find a cheaper ebook make sure it's not a rental.)
Even when you find a cheaper ebook, chances are that it's still not cheaper than a paper copy. How much is that paper copy worth when you sell it back to the bookstore? Subtract that and you'll get the actual cost.
So far as I can tell, digital textbooks are surviving for 2 reasons: college textbooks are frakking heavy and most college students don't know how to shop effectively. Price is certainly not a reason to get a digital textbook.
image via HarrisinMI