Whats a hobby/craft that you wouldn't expect that has an incredibly high ceiling either monetarily or in sheer skill?
For example, I'm sure the average joe doesn't know just how expensive calligraphy pens can be, or how deep the rabbit hole goes on video game speedruns.
Woodworking and collecting old tech. Both my hobbies / crafts, both started very cheap and very little, today in my workshop there's upwards 60k only in machines, not counting the tools, if you want to have a working computer from the 60s or even 50s, you'll pay. And pay and pay. My advice: collect old cars. Or yachts. Cheaper :)
Woodworking is as expensive as you want it to be. Other than a good table saw (which some people would argue isnt necessary for true hand tool fanatics) equipment just makes things easier and there's almost always another way.