What is your "inexpensive" hobby that turned out to be expensive/ you gradually invested lots of money into?

I've had amazing luck with hobbys that should be expensive, but weren't.

Me & some friends have a small computer museum. We collect minicomputers & workstations. (Stuff used in science & academia.) We have computers dating back to the early 60s. But we started in the mid 90s, when NO ONE was interested. So we got everything for free. (Well... for the cost of renting large trucks.)

I'm a photographer. My DSLR is old, from just when DSLR's were getting "good enough" at a reasonable price. I bought it used when it was already "obsolete". And then someone gave me an exotic industrial camera they had at work which was "broken". It was too broken for industrial use, but works fine for studio use. I had to build some hardware & write all the software to use it, but... the results are fantastic. It blows away my DSLR. (But uses the same lenses!)

My library has probably cost a lot, but that's spread out over 40 years, so I don't notice it. (Also, I worked in a used bookstore for a bit, and that's a good way to get a lot of books CHEEEEEEEAP. Employee discount? Yes. Discount on books in the back that are slightly damaged and unsellable? YES.) And I've occasionally sold a rare book, so that offsets things.

Etc.

(Note: my home computer collection spans ten full-height racks. A few of those are on loan from the museum, but most are mine. Spent close to nothing on that. Somehow.)