variant
Adventurer
THose other RPGs costing 50 or 60 bucks only require 1 book for the complete rules. Pathfinder is PHB and DMG and is large. The other (clones) D&Ds are also cheaper than WoTC apparent price. Also inflation doesn't really explain a $35 to $50 jump since 2008.
Hackmaster is three books, Player's Handbook, Hacklopedia of Beasts, and the upcoming Gamemaster's Guide. Others that require only one book are simply barebones RPGs with less content. In a lot of those other RPGs, while a whole game is contained in one book, it doesn't mean it has the same content of the three book set of other games. A lot of that content is cut and put in supplemental books which you have to pay for to get it anyway.
The question is whether the amount of content in the book justifies the price. My hope is that we get many more pages of content and make the PHB a more insular product. Most players shouldn't need more than the PHB.