For the collectors out there who want to be complete, the process will get expensive (the need to buy all the product). I'm not a collector, though, so its not an issue for me.
I don't buy every product either - I have all the hardcover books, just the Skull & Shackles set of 6 mods, a lot of the map folios, a few of the city booklets, the three booklets on Demons, Devils, and Daemons, and a couple of alternate publisher books like the Psionics book and the Tome of Secrets.
I've never seen much use for the Pawns, and I generally create my own worlds, so most of the Golarion stuff is basically useless.
I have enough 3.5 type books to choke an elephant, though. There's so much good material in them that changes 3.5 original rules that some people may not know where to start. I have =very= few modules and/or adventures - I've been creating them for years.
"The" GURPS GM