The $99 upgrade is to Mathcad 14. If you already have Mathcad 14, then I don't think this will help you.
If you don't have Mathcad 14, this is a great offer. You can run Mathcad 14 with other versions of the software, and the offer includes all the Extension packs and E-books. I posted the link to it in the Announcements forum.
Mona