This article shows an example breakdown of the costs involved in making a hardcover book:
Based on a list price of $27.95
$3.55 - Pre-preduction - This amount covers editors, graphic
designers, and the like
$2.83 - Printing - Ink, glue, paper, etc
$2.00 - Marketing - Book tour, NYT Book Review ad, printing and shipping
galleys to journalists
$2.80 - Wholesaler - The take of the middlemen
who handle distribution for publishers
$4.19 - Author Royalties - A
bestseller like Grisham will net about 15% in royalties, lesser known
authors get less. Also the author will be paying a slice of this pie
piece to his agent, publicist, etc.
This leaves $12.58, Money magazine calls this the profit margin for the retailer, however when was the last time you saw a bestselling novel sold at its cover price.
Salon.com also has an interesting post on what makes books cost as much as they do. According to that article, the physical cost of the book comes from the quality of paper, the printing, and the binding. That accounts for about 20% of the book's final price. The rest covers the publisher's other costs (editing staff, promotions, etc.), distribution costs, and booksellers' profit. The author also gets a cut - typically around 10% or 15% of the price.
There are other considerations in book pricing, but they seem to have more to do with how the bookselling industry works than with the book format, so I'm omitting them here. The Salon article goes into more details, though, and there is also some interesting information here about hardcover pricing if you look past the political bent of some of the comments.
I wasn't able to find a detailed breakdown of the costs involved in making a paperback, so I looked at lulu.com - a self-publishing service - to get a ballpark figure. I ran their manufacturing price calculator on roughly the same parameters.
For a paperback, I used 100 pages of standard paper, black & white printing, US Trade size (6" x 9"), bound with the "perfect bound" binding. The manufacturing cost on that came out to US $6.50 per book.
For a hardcover, I used the same parameters except for binding. With casewrap binding, the cost was US $15 per book and including a dust jacket bumped that up to US $16.
So it seems to me that if we assume that marketing and distribution costs are roughly the same for all types of books, the production costs would account for a good part of the price discrepancy.