The short answer is, it is free - they’re asking for a contribution but most people would enter a custom amount of $0.
There’s a longer answer about what free means in different contexts and how that pertains to opensource, and a longer answer about how “free” things have led us to the internet we have today, but I don’t think you’re asking about either of those things.
I’m daily driving debian on a lenovo t490.
Can get one for a few hundred. With a dock and 2x 1920 monitors its just beautiful.