My DD10 needed one for 6th grade. I got it at Walmart a few weeks back. I think they were just under $5. Regular price. Available in several different colors. Not a great deal though if you have to four. I haven't seen any sales on the bigger binders.
The Avery brand are buy 2 get 1 free at Staples. These are the ones that I've always used in my office and are made very well.
My DD12 who's going into 7th grade needs 2 three-inch binders and 3 one-inch binders I'm still trying to figure out how they're going to fit in her lockers along with all the books.
I bought the highest grade Avery ones at Target for $7.50. I thought about the Staples deal, but he only needed 2, and I suppose I could have saved one for the following year, but I decided to just go with the dollar cheaper price at Target.
Thanks for the idea about Costco. They didn't have any online and ours is 45 minutes away so I checked BJs.com and got FOUR for $15, shipped. I don't even have to run down there! I knew you guys would com through.