I don't know the specific numbers and details. What I do know is that printing costs don't rise at the exact same rate as page counts. In some cases, you can pay less per page by printing more, but in other cases you can't. I know that it's cheaper to print books at certain intervals than others. (For instance, I know that for some companies, it was best to print books in page counts that were divisible by 8; 16 was better, and 32 even better than that. But I don't know if that's always the case.) And I know that there's a certain optimum profit-to-cost ratio beyond which it becomes less cost-effective to print larger books, even when there is a bulk discount. Sometimes the companies will do so anyway, if they really want to hype a book, but it's not necessarily the best decision in terms of pure business.
Then, of course, there are predicted sales numbers to take into account. WotC knows the PHBs will sell in greater numbers than anything else, whereas more specialized books are going to sell less. This, too, inflates or deflates the "reasonable" cover price.
My point? Printing's a wonky business. I've looked into it, a little bit, and I still don't understand it. But I do know it's far more complex than a simple "This book is X% longer, so it should cost exactly X% more."
(All this, of course, completely ignores the point that there's absolutely nothing wrong with any RPG company, WotC or otherwise, charging what they think the market will bear for any given book.)
