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.
Astronomy as someone wrote, but some other hobby can have high ceiling, like RC stuff, radio control ; sure as long as you stay on the road you start with a 1/16 brushless car for a few hundreds and can upgrade to 1/6 2 strokes behemoths for a couple thousands. Some people are in rock crawlers and it can become expensive too even for a hobby. When you go in RC planes, first the radio goes from a simple 2ch one to a computerized 16ch one, then the plane can go bigger and bigger, brushless powered, nitro powered, gas powered, turbine powered! And you basically have an RC jet costing thousands of $$$, you can see some crash on youtube, it's scary :-/