Frozone IN the $4 ($3.99) deal, is what most players are referring to when they mean he's cheap. Otherwise, the gem cost for him solo is in the 3rd or 4th most expensive package when just purchasing gems, and if you read around here or on the game wikia page, you'll see that gems aren't that easy to come by in normal game play. So when Disney offers packages for "premium" characters (Pluto, Rex, Maximus, and now Frozone), most folks jump on the opportunity for cost (and time!) savings.
I have all the characters so far in my game that I can purchase (Pluto, Rex, Boo, Maximus, and Frozone), as I'm not far enough to have gotten to Merryweather's game play. I bought gems to acquire Boo without a deal (since one wasn't offered to me), but bought Pluto, Rex, Maximus, and Frozone with their respective offered gameplay packages. I'm sure Gameloft has analyzed the spikes in in-app purchases when packages have been offered, and probably saw a significant rise in gem purchases during the Mother's Day Sale that was offered back in May.
Ok great thank you! So I guess I'll just keep my eyes peeled for deals. Still trying to decide if I want Frozone or not!