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.
You can get a little table top router and some simple software for a couple hundred bucks. You can go deep into it. Building a custom machine, writing your own post processor, dialing in you CNC to insane levels of accuracy and precision, adding a 4th axis, engineering parts and projects, it goes on. It basically combines robotics, design engineering, and manufacturing engineering all in one hobby.
To piggyback off this, hobby machining in general. You can buy a small lathe pretty cheap but add some decent tooling and oh you need a very fancy tool you don't have? Hand over more cash. You want to make a part that won't fit in your small beginner lathe? Time to fork out for a bigger more expensive one. Oh a mill would be better for that part? Those kidneys could get a basic mill. Oh you need more fancy tooling for the mill now...