At least in my observation, DMs tend to have a stronger commitment to the game than non-DMing players. Of course, there are plenty of totally hard-core players too, but in many groups there are some players who don’t purchase a lot of supplements. Instead, their decision is simply whether or not to buy a PH, and that decision is based on how useful it is, how much they like D&D, and whether they can afford it.
To WotC, these players aren’t the core audience per se. In a gaming group of 6, WotC makes much more money on the 2 uber-geeks who buy ten books each than the 4 other players who buy 1 or 2. But WotC (A) holds out hope that some of these casual players become mass buying uber-geeks and (B) realizes that, for many of us uber-geeks, we need some casual players to get our games together. (Well, maybe not PirateCat – I suspect he would still find players even if his whole campaign moved to California.)
Because casual buyers tend to only buy the PH, PH sales are made up of a higher proportion of casual buyers than the DMG or MM. This probably means that PH sales are more prone to decrease with higher prices. In economic terms, PH demand is probably more elastic than DMG/MM demand. If WotC also believes this, they would price the PH relatively lower than the DMG or MM. Also, casual gamers help the rest of us play more, which makes us more likely to buy tons of supplements. Therefore, casual gamers have an indirect positive effect on demand. (A network effect, in economics.) Lastly, we shouldn’t neglect the PH’s role as a marketing device for the rest of the product line.
Economies of scale are relevant, but I think these factors probably dominated WotC’s decision to price the PH lower (per-page) than the DMG or MM.