My hobby is buying materials for projects and then not doing the projects.

Spending money on hobbies is fine, change my mind.

there has to be a list of hobbies one can try that cost practically nothing:

Solving Rubik cubes (a high quality speedcube is about 20$)

Crocheting/stitching (needles and yarn after cheap)

Writing (free)

programming

... (please expand if you have any ideas)

If you want to save money, don't get into bird photography as a hobby. Gear Acquisition Syndrome is fatal to your wallet.

"Turn your hobby into a business!", they said. "It'll be fun!", they said.

Piracy saves money if you're already paying for subscriptions. Self hosting adds another way to save money.

Nothing empties a wallet faster than ‘I’m just trying it out.

As an amateur radio operator, I can confirm this as factual. Over.

Software development is cheap, if you already own a PC, which is the most expensive part if you go with open source tools.

I got into self hosting a while back

EOL enterprise equipment can be very thrifty. But if you ever need something specialty that isn't available in the second hand market, good luck.

Pssh, only if you aren't turning all your hobbies into side-hustles.

What are you afraid of, besides completely burning out and probably still losing money anyway?

Warhammer 40k has entered the chat.

Well if I stop buying fishing gear, at some point it'll be financially worth it. I mean I've already "paid off" the license for this year. Yup... Ignore the brand new rod and reel I got for black Friday, it was on discount. Or the fact that I got a gold membership discount at the local tackle store because I spent enough money in a year.

I live in Canada, do you know how much the standard video game costs here now? Like $90.

That's how expensive skydiving hobby is. Unless you're an instructor. Then you get paid to skydive.

If you are looking for an endless money pit, take up the guitar.

My hobby are maths and programming, both fairly cheap. Other than the cost of getting a computer of course (and maybe some maths textbooks if you feel the need)

Cries in synthesizers.

At least there are a bunch of great budget options these days. Still not cheap though.

Looks at ever growing pile of retro handhelds, tabletop miniatures and BDSM gear

Surely, it cannot.

I barely spent any money on my hobbies this year. I'm not some kind of sucker!

I merely spent thousands of dollars on materials and tools to build the supporting infrastructure for them!

I also have some very expensive computer parts waiting to be assembled. But they aren't for any hobby of mine! They're for the kid so we can play stuff without him using a computer twice as old as he is. 😉