The single-threaded CPU performance and the GPU performance of Nexus 9 runs circles around TabPro 8.4 (not to mention the fact that the Nexus 9 has a true 64-bit CPU running on a 64-bit enabled OS, and a much more forward looking GPU architecture too with support for Unreal Engine 4 and all the latest and greatest API's including Android Extension Pack). So you get what you pay for, so to speak.
Note that Nexus 9 will ship with the Android "L" OS and with some 64-bit native apps such as Chrome, Gmail, Calendar, Google Play Music, etc.
You see, not everybody is a gamer. The TabPro does just fine for stuff like browsing, reading, and moderate gaming. And it's basically half the price.
That's the kind of niche the Nexus 7 used to occupy: quality budget device, cheap, but pretty good for a lot of tasks.
On the other hand, if I want to have the best of everything, I sure as hell wouldn't choose the Nexus 9 over the iPad Air 2. It's just a tweener device that occupies a space that hardly anyone cares about.
And btw, what the hell do I care whether my Mail or Calendar App is native 64 bit? That's just ridiculous. Even full fledged desktop applications like Office 2013 are usually still 32 bit (I know there's a 64 bit version, but just about everyone uses the 32bit one). So why the heck should I care whether my calendar app is "native 64 bit"???