The reason why you pay less for an upgrade is you are paying only for
new features introduced since the previous version. This is the way the music software business has worked for at least 30 years. With X3, Melodyne was a new feature so the upgrade cost paid for a Melodyne license.
With Platinum, Melodyne is no longer a new feature but now part of the program. Similarly to X3, the upgrade cost for Platinum paid for the new features (VocalSync, new Control Bar, MIDI Paintbrush, AD 2, Mix Recall, etc.) as well as more new features and bug fixes for another year.