Modular synths, eurorack is where you find the most accessible modules than the other formats. Sometimes you go and spend 600€ in a module without batting an eye.

Also you have to count the case, patch cables, etc.

It gets expensive quickly if you can't fight the GAS (gear acquisition syndrome)

Also it is a musical instrument so you need to practice many hours to play it affectively.

It is really cool, I do enjoy myself playing with my modular, but would love to have more time to spend with it.

Mountain biking.

Have you seen the price of polo ponies lately?

Camping. Whether it's at a campsite, where a family might spend tens or hundreds of thousands on an RV and all the gadgets in it, or deep in the woods, where an ultralight backpacker might spend thousands of dollars upgrading perfectly good gear they already had because it could save a few ounces.

To be clear, camping is actually really accessible, and few people go THAT extreme with it. Just... no matter what budget you set for it, there are ways to spend it. :P

I imagine taxidermy is super difficult

You can get started homebrewing cider or mead for like $50, especially if what you're brewing isn't carbonated so you can just store it in whatever. You can also buy home garage versions of fully automated brewing and canning lines that will run you into the tens of thousands, not accounting for consumables. It escalates fast.

Billiards

Amateur astronomy, you can start with a modest Dobsonian then it get can very expensive very fast and you need to understand celestial coordinates, ccds, optics and such.

Telescopes

Astronomy as someone wrote, but some other hobby can have high ceiling, like RC stuff, radio control ; sure as long as you stay on the road you start with a 1/16 brushless car for a few hundreds and can upgrade to 1/6 2 strokes behemoths for a couple thousands. Some people are in rock crawlers and it can become expensive too even for a hobby. When you go in RC planes, first the radio goes from a simple 2ch one to a computerized 16ch one, then the plane can go bigger and bigger, brushless powered, nitro powered, gas powered, turbine powered! And you basically have an RC jet costing thousands of $$$, you can see some crash on youtube, it's scary :-/