one bag of Stubbs will only fill up one chimney.
I think you may be buying the smaller bags
. I have a weber chimney (large not the mini) and I used Stubbs this weekend. I filled it all the way up twice and 3/4 of the way up once and still have 1/3 of the bag left.
You can buy a 16 pounds bag of Stubbs at Lowes for $8.99 regular price, thats pretty strong.