You pay one time to download it. If you decide reinstall the app on the same device or install a copy of the app to another device iTunes will ask you to buy it again. Once you press “Buy” it will tell you that you already purchased the app. You will NOT be charged twice. You pay once.
If it is is a universal iOS app, you can install it on all of your iPad, iPhone and iPod touch devices with no additional charge. For instance, we offer Checkbook HD and Debt End as universal apps.
If it is not a universal iOS app, you need to buy a separate app for your iPhone and separate app for your iPad. For instance, we offer Money for iPhone and Money for iPad as separate applications.