Well, I don't really care about sales that much, though, even the girls (Kalafina) themselves, they said it's okay if you're just buy one copy of their physical release.
There was an interesting story about them and the fan during the release of heavenly blue (as a re-cut) single. They are doing a promotion in NicoNico and during the show, they read some fans comments. There's a fan saying that he wanted to buy more than one copy for this single, because they want to support them. Interestingly enough, Keiko answered that comment by saying that one copy will do, and Wakana added that as long as our song can reach their heart, it's more than enough and I also heard that Kalafina gained more recognition after this. (Have I told this story before? I think I did.)
Anyway, speaking about sales, their sales numbers are really determined by the popularity of the tied-in anime. This is one of the most trusted source:
http://anisonsinger.blogspot.co.id/2.../kalafina.html
Madoka Magica was really popular and it helped to boost Magia sales, and I think Kalafina popularity as well. All the singles related to that anime (Magia, Hikari Furu, Kimi no Gin no Niwa) is really helping. That being said, we can conclude that Kalafina sales numbers depends on the tie-in.
I hope this is an interesting information (and open for discussion).