You buy your fire tv device from Amazon for $40.
As for putting Kodi on the stick, that's where it gets complicated if you're not familiar with what ADB is.
I think PC + HDMI cord or a Chromecast is a better option.
There are a few installers that make using a FireTV device with ADB a much simpler one click install process on OSX and Windows. You don't even see the command line. Just enable ADB in the FireTV settings, enter your IP in the program and click install.
A PC+HDMI cord only works if you're willing to lug a computer around, run a long ass HDMI cord, or build an HTPC. All of those can be an inconvenience compared to a simple device that's connected to the tv. Then your have to factor in control devices and the such.
The Chromecast also doesn't compare as they typically have different use cases. I have 3 FireTVs and 2 ChromeCasts and they don't have that much in common when Kodi is involved.
I think the FireTV is one of the most underrated deals out there for how easy it works with Kodi and how powerful it is. I would suggest getting a FireTV over the FireStick for the additional horsepower though.