I bought a 2013 Nexus 7 open box for around $140 from Amazon to use as a monitor. The Shield looked really tempting but since this one is coming up on its first birthday, I am betting that Nvidia will be releasing a follow-up device that runs on the newer Tegra X1 platform within the next couple months. Buying a new Shield right now would probably result in disappointment very soon unless you only use it for flying.
With a 1080p screen, no manufacturer crapware bogging it down, and a quad core processor, the 2013 Nexus is probably the cheapest route to get more than enough power for the P3. This summer, when the next Shield Tablet is released, if that's tempting enough, I can always throw this tablet on ebay or craigslist, get a little money back, and pick up a new tablet.
With all that being said, I had hoped I wouldn't need a dedicated tablet any more after I bought a Surface Pro 3. It looks like iOS and Android tablets still win a few battles... Maybe one day there will be a native Windows 8.1 (or 10) DJI app (I realize I could run Pilot in an emulator but sadly the SP3 also lacks a GPS chip).