Obsidian is free to use, mostly
You need to pay for Obsidian if and only if you use it for revenue-generating, work-related activities in a company that has two or more people. Get a commercial license for each user if that's the case. Non-profit organizations do not need commercial licenses.
For all other uses, you can use Obsidian for free forever.
To support our development, you can pay for one-time Catalyst licenses to get early access to new Obsidian versions, but it's completely optional. You can contribute more via Obsidian Unlimited if you wish.