Knowing something of printing from various contacts in the industry (both printing and RPG), I'll throw in a bit of info. Of course, WotC might not be following this philosophy (and I'm sure The Rouse can correct me if I'm wrong) but...
Most entry-level products (for D&D, the PHB...but not the DMG or MM) for RPGs are considered "loss leaders." What this means is they actually take a hit on profit (in some cases, actually losing money per book sold) for that product in the hopes that people will spend money later on, where the real profit is (sourcebooks). This is why said entry-level products are considerably more economical than other products in the line.
(Incidentally, it works similarly in the video gaming industry; most manufacturers take a hit on the console itself because the sale of the games more than makes up for it)
Also, somewhat related is the fact that (most) printers charge per unit (I forget the exact name). Each unit = 16 pages. This is why most of your books' page counts are divisible by 16. (320 pages = 20 units; 224 pages = 14 units and so on) If, say, the DMG were 230 pages, the printer sould still charge the 240 page rate. (PoD might break this pricing structure) That's why you hear about books that are sometimes cut for space reasons; if they can't fill up 16 more pages worth of material, then it's not very economical for the publishers to include that info.