The Kindle Fire HDX runs Amazon's proprietary version of Android on a 2.2 Ghz Snapdragon 800 CPU with 2GB RAM, 16GB/32GB/64GB internal storage, dual-band Wifi, enhanced audio. It has a 7" screen and costs $229 from Amazon, though you can find it for less if you shop around.
The Fire TV runs a modified version of Android on a 1.7 GHz quad-core Qualcomm Snapdragon 8064 CPU with 2GB RAM, 8GB internal storage, Wifi, Bluetooth, and more. The Fire TV retails for $99.
Something tells me that Amazon is not selling all that many Fire TVs. This much hyped gadget launched in early April 2014 with a $99 price tag, and two months later it is being sold at an effective retail price of $20. Sure, Amazon regularly offers sales on their hardware and there are some obvious synergies between this tablet and the Fire TV, but I think this deal is too steep of a discount and it came too quickly.
Admittedly, no one knows how many Fire TVs or Fire HDXs have been sold, but it strikes me as a little unusual for Amazon to offer such a steep discount during an off-season.