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 😁