I have already purchased TargetScan in the Apple App Store, can I download it for free from Google Play (or the other way round)?
Unfortunately you will need to purchase the App again. This is the same as with all other paid Apps, Movies, Books, and Music. The good news is that the licence on the old platform remains valid for life so if you ever switch back you will be able to download it for free.
In order to transfer your data please create a backup file on the old device and restore it on the new device.