This one is a tricky call. With Modo Drum 1.5, they were offering new drum kits but no new features. So its about $26 per kit. And I am sure if you wait a bit, it will go on sale to make it $10 to $13 per kit which seems quite reasonaable. If you are a heavy MODO Drum user, which I am, its fine. Its a lot less than what XLN or Toontrack charge.
For this Mixbox 1.5, it's tricky. They basically selling you a few extra modules and presets. My guess is the modules software is the same thing as you get in SSR so you are paying for the privilege to use it in Mixbox. Where is gets tricky is, they already released other modules in the past as free update. So what makes SSR worthy of charging extra? I too think this should have been free for current SSR owners. ?
https://www.ikmultimedia.com/news/?item_id=10486