That's not how it works though. It's all about odds. A chest has the same odds whether you paid real money or earned gems. So, if you had saved that iTunes card and just bought the one chest, you still most likely would not have gotten Anna at all.
But I agree with you. Doesn't seem fair at all. I think that if there really are people out there willing to spend $10 for a digital character with limited game play, for a game that might not even be around in a year or two (VMK anyone?), then they should have it. Gameloft should consider possibly running that kind of promotion. At least once, to see if it's profitable. "Pay $X for X character and get X gems free" Because the current probability offer borders along the lines of gambling, which for a game that has a large child fan base, isn't very "Disney."