I also like using Carbon to matify and intensiy my black eyeliner whether I apply liquid, pencil or cream liner.
MAC's refill pans retail for $14 CAD. The benefit of the pans is that you can customize your own palettes.
For $18 you can get the same eye shadow in a small compact. If you don't use a lot of eye shadow you can get this one to simply throw in your kit.
Overall, even though it may get a little pricey to buy numerous refills, Carbon is definitly one that will become a staple in your collection.