That brings up the 7th edition for me, while the app is based on the 9th edition. The 9th edition hardcover price is actually cheaper than the 7th edition, at $154.00:
But the "in-app purchases" section of the App Store page lists the online price as $119.99, so online still works out substantially cheaper than the new hardcover. I'm not sure where the higher price came from.
Regardless, you can do even better at other sellers or used if you're willing to get the 7th edition of the hardcover. At other sellers linked from Amazon the price goes down to about $100, and the used price is around $5. (Amazon does make this more difficult by displaying a lot of noise in the form of other books named "Biology" by different authors, even though the URL specifies "Biology-Peter-Raven".)
Edit: App Store web page for Inkling with textbook and chapter prices:
Honest question: how do you get money back from an used book over there? Around here it's unusual to sell school books when they are not needed, people I know just give them like I did so I'm wondering if it's common to sell them or if there's some program which let's you return the book for some money (which could be an interesting business if it doesn't exist yet).
All through University, I'd buy my books used and sell them back to the student bookstore at the end of the semester. Pretty much everybody did this. You generally get about 75% of the used price back.
Towards the end of the semester, when beer money was running low, there were always tough choices to be made. A lot of those books would have been useful to have around today, but pressing needs prevailed.
Yeah, there were always a few books that passed the keep-test for me. The best was this lovely Gartner's Art Through the Ages visual history that was a quite a challenge to lug to class but has had a permanent home under my coffee table for several years now.
[1] http://www.amazon.com/Biology-Peter-Raven/dp/0072921641