Zardnaar
Legend
This was explained by @delericho in the OP: "Prices are for the second printing onwards".
That tells us that the inflation-adjusted price of PF has dropped since 2009. It doesn't have any bearing on what the inflation-adjusted price was in 2009.
Well if you can buy the core books of PF for $90 vs $150 for D&DN thats not a great deal. The $60 at launch deal ($81 now) probably helped catapult early momentum for 3rd ed so being cheap helps.