The Nvidia Shield 2 might be coming as soon as next month, though I haven't seen any confirmations of a release date or even its existence yet. If it's priced like the original, it should be $300 for the lower storage, WiFi option (16GB last time). It would almost certainly be the best gaming tablet for the price.
Otherwise if you need a tablet now, the Nexus 9 would probably be the best bet. It goes for $399 and has the same graphics chip (Tegra K1) as the original Shield.
You can also find the Nexus 7 (2013), Samsung Galaxy Note 8.0, or ASUS MeMO Pad 7/8 going for $100-$150. They are older and don't have specs as high as the Shield or Nexus 9, but worth considering if you want an inexpensive gaming device.