D&D. When I got back into it as an adult it was mostly because I could get into it for $0. I was dead broke at the time. I pirated the books downloaded the free basic rules 😉 on my trash find laptop and was good to go.
But man once I had money it turns out I really like collecting books and the D&D ones are not cheap. I do not want to think about how much I've spent.
The real flood gates opened when the license controversy happened and I decided to try other RPGs. Multiple core books of multiple RPGs gets expensive fast 😁
I’m a few hundred dollars in and I’m actually on my way out thanks to their grossness this spring. I’m too slow to have gone for many modules, but rules books were super fun and thus a money sink. I’m trying to move my group over to PbtA (cheap by comparison) or Genesys (have you seen the cost of those DICE?!).