The slip-cased set of D&D4e PH, MM and DMG. I think it was about £70. It was one of my biggest disappointments: I was about 20 pages in when I realised I really didn't want to play this game. I gave it away.
about $600 for my 4E on-line tools subscription until 2016: worth it for the usefulness and enjoyment, but unfortunately it left me with no tangible product afterward
I bought a game store in 1998 for ~US$60k, and after the loan was paid off, it probably "cost" me US$100k. RPG inventory on purchase was probably ~US$7-10,000 at cost.
But I didn't count that.
I limited my answer to role playing games only and I think the most I've ever spent for a single book is $60 so that's how I answered. Of course for D&D 3rd edition it cost $90 for all three of the main books.
I voted 50-100$. I refuse to pay more than 60$ Canadian for a singe RPG book. But then I turn around and buy the whole set of Modern AGE + Fantasy AGE books within 12 months...
My son got a cool book bag for Christmas for about $75. It has some cool slots for books and such with a foam-lines top for minis and pouches on the sides for dice and pens.
For myself, I bought one of the $50 adventure books.