Horses. Lessons at $35-$60/hour, boarding horses, vet bills when they were sick, farrier, tack and supplies, hay and feed, training, horse trailer and truck to pull it. We recently bought 10 acres so I can have horses here...so as you can imagine our mortgage is higher than an equivalent house on a small lot, and our commute is much longer. We're doing expensive projects like fences to be able to keep horses here. I'm not sure I even can add up what I've spent on them over the years, but probably close to $100K over 23 years. Some of those years I owned horses, some I only took lessons, some I half-leased a horse. I even kept mine cheaply as horses go - self-care pasture board, not doing a bunch of shows, doing my own vaccines and things like that.
I kept my two horses (who each cost less than $1000 to buy) for a total of about $6-7K/year, when I was making ~$30K. I was single, lived in a tiny studio apartment and walked to work. Other than the horses and good food I lived pretty cheaply, funded retirement, and didn't go into debt. I only did local schooling shows that cost $10 or so per class, and dressed neatly but not fashionably.
On the other hand, I know people with $30K + horses, who pay over $2K/month to board and train them, plus pay for shows. Some big shows cost thousands of dollars in entry fees, trailering, stall, grooming, braiding, coaching, show clothes, and all the other attendant expenses. Horses are never really a cheap hobby, but they don't have to be that expensive.
For a while, we also owned a 30' sailboat, which was my husband's hobby of choice. Also very expensive. We had it shipped down the coast, and paid $300+ per month in slip fees. It was fun, but we hardly ever had time to use it. I wasn't going to push him to sell it, but I did not object when he decided it was time.
