Why do you say it "should" be balanced this way. Is that your gut feeling? A level you have found most profitable through empirical evidence? Or some other reason?
Not an attack I'm curious as to how one arrives at the level of in app purchases. Reflecting on it myself I feel there is likely a maximal profit level that one can achieve that you should go for. Do you think differently?
The game should be balanced in such a way that people who really don't want to pay for it, can finish it, while people who are "lazier" could pay for some stuff to get them over some levels. But even then it shouldn't be like $99 for a game, except maybe for 0.1% or less of the players. For the vast majority of those who play, it shouldn't cost them more than $5-$10.
I think this way you get a game that is very popular in downloads due to it being free to try and play, and it should also make at least as much money as if costing $5-$10, but because it's free to try, and it could get addicting, and also because some players may spend tens of dollars on it, then it also has the potential to make a lot more. Just don't make it so anyone who wants to finish it must spend tens of dollars on it.