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.
Telescopes don't really get too crazy until one wants to get into astrophotography (assuming we want an APO at that point, not before). At that point staying in the four digits is an accomplishment in itself...
But I did learn something when I bought a binoviewer for my telescope, I had to buy double the eyepieces and that was a bit hit that I didn't fully think out