iPhone apps for Arduino control


I want to make a simple app for Apple devices that controls the position of a motor, I have everything under control apart from the how to make an app part.

So my question is, how do you go about starting to build idevice apps for controlling Arduinos?
I understand that it might be expensive to get started?

I know there a few apps available already but none of these are really useful to me.

Thanks a lot,