The use of the present tense might be appropriate if you have not already bought the book. Assume that your friend wants to buy a book and the price in your local bookshop is $5. You also want to buy a new book and the price in your local bookshop is $25. You would then be able to say "The new book I want to buy costs five times as much as yours". I would not be very happy with "My new book costs ..." in that context because that makes it sound as if you have already bought the book. Once you have bought the book, it becomes "My new book cost ..."