Here the "standard" initial biology book is the Curtis: https://www.amazon.com/Biology-Helena-Curtis/dp/087901394X

It's targeted to first year biology students, so it's light in the math part.

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:

http://www.amazon.com/Biology-Peter-Raven/dp/0077350022

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:

http://itunes.apple.com/us/app/inkling/id379351586?mt=8

I can only see the price of the biology book, $149.99, which is cheaper than the paper one, $178.75 on Amazon[1].

[1] http://www.amazon.com/Biology-Peter-Raven/dp/0072921641