I already bought USB Disk Pro on iPhone, why does it now costs something?

The app you find when searching for USB Disk Pro in the App Store is a new app (which adds full support for the iPad but which otherwise has the same features).
To download the version you bough, open the App Store app, select the "Updates" tab, press on "Purchased", scroll up to reveal a search field, search for USB Disk then press the cloud button next to the name of the app. Once installed, updates for the app will appear like other apps in the "Updates" tab.