If it's an app that's universal - i.e. it has a '+' next to it in the App Store, then it will automatically adjust to the different screen resolution of the iPhone. You can simply download it again to the iPhone - assuming both your iPhone and iPad are on the same iTunes account - and you won't be charged again.
If it's not a universal app - no '+' symbol next to it in the App Store - then you'll have to download an iPhone-specific version and, unfortunately, pay for it again.
Tim