Crystal Guarantee?? Me Thinks
If there are 6 star champions in the crystal, the guarantee should be at least a 4 star. If the crystal tops out at 5 star champions, then sure the guarantee of a 3 star champion.
It makes sense to guarantee two levels down on the maximum star champion able to be received. I think that would be fair when you're asking for $ for crystals ... two levels below the maximum star champion in the crystal should be the "guaranteed to pull at least" part of the draw.