Cost per user acquisition of iPhone game apps really depends on the game and the subject matter of the game.

Here's some general data on the topic of cost per installs:

iOS app CPI Globally – $1.24
iOS app CPI in US – $1.64
iOS app Cost Per Loyal User in US – $2.78
iOS app CPI in Europe – $1.4
iOS app CPI in Asia – $0.90

Getting a user to install the app is one step. Getting them to register is the next step.

I will need to know more about the game to speak more deeply to the details, but if it's a general game that's fun to use, and looks appealing, my hunch is you should be able to hit to these CPI numbers.

How to acquire the users is the next strategic question.