1. Get the ISBN number (The International Standard Book Number (ISBN) is a unique numeric commercial book identifier) of the book from any of the online book stores or through a search on google.
2. Type isbn.net.in/"the isbn number"
3. This would automatically search from 15 online sites and give you the price at each site for the same book.
4. Buy from the site giving you the least price.
Warning: But, it doesn't seem to be working for all the books. Anyway, a one min work can save you a hundred or so.