My principal hobby is soaring: flying sailplanes (aka gliders - which despite the name are distinct from hang gliders). This definitely ranks as expensive: a decent glider can cost over $100k (but they tend to hold their value well). An aerotow launch (typically one per day for an experienced pilot) is ~$65.
Minor hobbies include 3-D printing, birdwatching and Via Ferrata (basically, hiking on a vertical cliff with a safety harness). None of these costs more than ~$1000/year.