Yes, I’m not getting rich with a $9.95 Blender Add-on.
Development is more than source-code, this value is very important to maintain support for the Batch Bake users.
And the Texture Tool (the texture denoiser) is not an add-on and it’s not linked, it’s a stand-alone software and can be used without Blender too, like an external render engine.
I have contributed to some open source projects on GitHub, but Texture Tool can’t be posted on GitHub.
I understand that for some people is hard to buy software or other goods (look, I can’t use the $125/month Unity Pro or Photoshop). That’s why I offered it for free to Odilkhan Yakubov, and is one of the reasons why it is so cheap.