Two non mutually exclusive things could be going on there. 1) The stuff where the price isn't going up already has high margins built in. (Kind of how restaurants make the most money on alcoholic drinks, not food) 2) The stuff that's going up is subsidizing the stuff that isn't going up. That is to say that Paizo expects they'll sell more rulebooks than 64 Page adventures. So they can charge more for those to keep the price lower on the 64 page adventures - maybe even at a loss. (Back in the day Best Buy would sell CDs at a loss to get you in the door to buy the stuff they knew they'd make money on)