You do not need to pay anything. The $19 is only required if you want to distribute your app to other people through official channels.
Here is the proper, factual explanation regarding the free aspects of Xbox Developer Mode. xbox dev mode free
| Action | Cost | |--------|------| | Install Dev Mode app | Free | | Switch console to Dev Mode | Free | | Run your own code/emulators locally | Free | | Get a developer certificate to publish | | You do not need to pay anything