Yeah, that's right, but the discount price isn't the same for all Bests.
Ayu's and e.g. the one from TRF (also Shuizka Kudo's), which are in front, have the smallest price from the ones I checked. Kobukoro's e.g. is still above 1000 Yen like some others. So the reason is more, they're generally popular artists and have a good price at the moment.
Every Little Thing is impressive - still over 2000 Yen and having a great position nevertheless.