Running. Not as expensive as a lot of the things posted about here, but my shoes cost ~$150 and I have replaced them a couple times a year. I’m planning to get in to trail running soon (as opposed to running circles in my neighborhood, so now I want to add a running vest and a GPS watch, which is not cheap.
Considering that in theory all you need to run is your body and an open space, I feel like I have spent a lot of money.
EDIT: I forgot the ~$140 bone conducting headphones I bought! I for sure feel safer with them than my old headphones though, since I have been doing almost all of my running till now on the road.