I'm sorry but I'm failing to see how having more intro and credits pages, table of contents pages, and index pages along with twice the hard covers and binding helps with cost. Maybe there's some discount or something I've missed but right now, I'm sorry. I don't get it.
This is all
speculation but...
-A 400 page book means 400 pages of possible errors so there's probably some cost adjustment for dealing with that annoyance.
-A 400 page book is going to take longer to print simply because there are more pages. And if the printer's life is anything like mine the print jobs will finish at the most inconvenient time, like at hour 7.5 of an 8 hour shift so you can't start the binding process on the same day as the print run. That in turn leads to more labor costs.
-A 400 page book will probably require more setup in the printer than a 200 page book, just because it's longer. If I'm not mistaken* the printers aren't like the ones you and I have access to in our homes or even a print shop. They're more like newspaper presses where you have to build the job
just so or it's a total writeoff. (See this
Imposition article for more info.)
-A 400 page book may well require special handling or tools for sewing the signatures together because it's thicker than a 200 page book. That can be another cost passed on to the customer (CGL) by the publisher, especially if the 400 page books are a tiny part of their publishing business.
Conversely, the 200 page books may avoid all those issues, so there's a lower cost to the publisher. That in turn means a lower cost to CGL, which (hopefully) means a lower cost to us.
As a different metaphor consider what a pain in the neck it can be to store documents printed on Legal sized paper (8.5" by 14") when everything in your office is sized to only handle Letter sized paper (8.5" by 11"). It's just different enough to be annoying. Apparently the 400 page books are Legal sized papers in a Letter sized publishing house.
*And I have been before, even as recently as two weeks ago.
