Yeah, I can do math. I might pay, say $3, for a guaranteed Figment. $3-5 is a small amount, easy enough for me to be a completist about the emojis, and not risk breaking the bank. But right now, I can buy 1000 gems for $21. That gives me 5 diamond boxes, each with only a 10% chance of getting Figment - or, just over a 50% chance of not getting him with any of the 5 boxes. That's not good betting odds for me, so I pass on buying any gems.
TL;Dr - charge me $3 for a guaranteed new emoji, I will do it. Charge me $21 for a 47% chance of a new emoji? Hard pass.