Flight simming. Started out with a cheap joystick. Now I have an expensive one, throttle quadrants, rudder pedals, a vr headset and I've built myself a button box and a flight seat. And I'm now I want a helicopter collective. Oh well..
Aircraft, scenery, support software like Navigraph, it all adds up. Fortunately aircraft and scenery are “buy it for life” and anyone who tries otherwise is liable to have rocks thrown at them.
VATSIM is free however, and that’s part of why it’s so great.