No, you don't have to pay again if you want to install Jump on your other iPad, iPhone or iPod Touch.

Lets say you've already purchased Jump on your iPad and now want to download the app to your iPhone: Open up the App Store app on your iPhone and re-purchase Jump. This is important: Make sure you use the same iTunes account you used to purchase Jump earlier on your iPad. After the purchase is complete you'll be shown an alert telling you that you've already purchased the app earlier and that the download will be free.

Note: You can use the above method if you delete Jump from your device and want to reinstall it.