Here's an example of what I typically buy. This is my April / May order. And keep in mind, it's not complete, as I'm considering what else to add. (Probably more KoK adventures, a few KoK supplements, and some other stuff from other publishers.)
Affinity Games
Windhaven campaign book
Fantasy Flight Games
Spells & Spellcraft
Fiery Dragon
Queen of Lies
Green Ronin
Freeport: City of Adventure
Armies of the Abyss
Mongoose Publishing
Quint. Rogue
Quint. Cleric
Quint. Wizard
Necromancer
Rappan Athak 3
Privateer Press
Monsternomicon
Guide to Iron Kingdoms
Thunderhead
Bluffside
WOTC
Lord of the Iron Fortress
Bastion of Lost Souls
Deities and Demigods
Faiths and Pantheons
Dragon magazine, #293 on up
Dungeon magazine, #91 on up
Masters of the Wild
On average, I'm guessing I'm spending about $150 - 180 per month. Of course, I buy primarily from TalonComics, so (what with the lack of sales tax, whew!, and the fact that if you spend more than $150, you get free shipping in the U.S.) I'm saving about 21% over buying non-discount locally. Otherwise, my list wouldn't be so long.
[edit: had to format the list] [edit: dang it, you CAN'T paste a formatted list]