The app itself is entirely and completely free
What is not free is the hosting.
If you know what you’re doing and follow the instructions, you can install it on any server that meets the requirements, including a localhost on your computer for testing it out.
I find that the Discourse Team and other members do a good job of Support at the meta site
But NOTE, if you decide to go your own way and deviate from what is officially supported, you will more than likely be “on your own”
That is, it might be possible to run the app with less RAM or with a different database etc. But if you run into problems because you didn’t follow directions, you can’t expect others to bang things into shape for you when things run afoul.