If you don't know about eBookPrice.info its a search engine for eBooks. It will show you which formats each store has it and their prices.
I wish I used it today when I paid $7.99 for a book at fictionwise.com and later found it cheaper at Sony Connect and Powels and another bookstore I never heard of before.
So I am including a link to install eBookPrice.info directly into the Mozilla search field.
Enjoy and happy book shopping.