I honestly don't mind this.

They're already gouging everyone for gamerpics and themes, you've at least gotta be consistent. What I would ask is that pieces would be around 15-25MS. I would pay that. But what would be an ever better idea is to get developers on board for this, and while they create achievements for their game, they create several pieces of content for these MS Mii's that would be unlocked during the course of playing through a game. Possibly at the same time as you unlock achievements.