After pointing out how it appears as though Marvel charges more for their more popular hardcover collections (using the theory that if you'll pay X for a Ms. Marvel hardcover, then you'll certainly pay X + 5 bucks for a Red Hulk hardcover), I thought it only fair to take a look at DC's hardcovers, for comparison's sake.
And while it does not appear as though they are doing the same "charge more for more popular titles" approach as Marvel, their prices do have a similar sort of "we'll charge whatever we think people will pay" approach.
To wit, among their October releases, for $19.99, you could have a premiere hardcover (DC doesn't actually use the term "premiere hardcover," which is to their credit) of Detective Comics, collecting six issues.
Or you could have a trade paperback of Batman: Confidential, also collecting six issues.
Or you could have a hardcover of Green Lantern collecting seven issues.
Or you could have a trade paperback of Batman Underground, collecting NINE issues.
In other words, DC appears to figure out a price that people will pay, and charges it, with content being a secondary consideration - "People seem willing to spend about $20 for a book, so let's try to make most of the books about $20."