If we try for a more apples-to-apples comparison, and look at 4e module sales, the situation becomes even more murky... If "the game" is doing well, then why aren't the mods selling? A canned scenario is closer to a movie/vidgame in that it's "prepackaged fun" as well as "cheaper" (the argument that they should be priced in the $15-18 range, rather than $25-30, is a separate topic, I guess...). Less investment, less frustration, and hence a better indicator of a "recession hobby", yes?
As for comparing the sale of something like a PHBII to a module:
(a) Only the DM would buy the module, but the players, at least some of them, would buy the books that are for players (PHBs and Power books)
(b) Many groups that would buy modules would also buy the rulebooks as well. This is, I think, the real reason that module sales would lag behind books like PHBII. You can play the game without the module, but playing the game without the rules is a bit harder. Making your own adventure is a bit easier than building a new class.
(c) Modules of comparable levels compete with each other, and there can only be one module at a time, more or less. With the rulebooks, they complement each other, and each player could be using a different ruleboook for their character. And with some situations, you may be using multiple books (multiclassing, the power books that supplement existing classes, race/class combinations that are cross book).
(d) while comparing a module to a video game or movie may be more accurate, it's probably a case of niche marketing. If someone is choosing to play a tabletop game like a pen and paper RPG instead of a board game, or video game, or going to watch a movie, it is perhaps as an alternative to the other options.
The market for any book after the core 3 would be people that purchased the original. To sell someone on picking up the PHBII is to sell them on the game of D&D and the core books. So any PHBII sale has a PHB1 sale, either in the past or at the same time.
With the core set of 4e, it was a sort of unknown property, and sales were likely spread over a longer period of time. With PHBII, people knew, to some extent, what they were getting going in. If they owned and used the core rules, their decision to buy the PHBII was likely made before the book hit, so most of the sales will be right away. The residual sales over time will likely be similar to, but a bit lower than, the first PHB.
