Wanking is a free and enjoyable hobby.

I've been there, but you can definitely find hobbies that cost nothing.

Hiking/walking/running is free and good for your mental and physical health (although if you dont have appropriate footwear, I'd stick to walking).

Music production is free (if you already own a computer) with free DAWs and free plugins. This is a great way to be creative, learning music theory is also free but not super necessary

Writing is free, this can include fiction, as well as journaling. I enjoy journaling but

Software hacking is free with ghidra and x64dbg, I've been working through crackers, which are pieces of software written specifically to be cracked.

Programming is free and the resources to learn programming are free. If you have no experience python is super easy to jump into.

3D modelling is fun to learn and again, free. Fusion360 will give you a free license for personal use, or there are other free options. I learnt this to 3D custom designs, and your local library probably has a 3D printer to use if you want to go that route.

Your city almost certainly has a free art gallery, I love spending days at the art gallery with headphones listening to beautiful music and looking at beautiful art.

Cooking can be a fun hobby, and although not free, it's money you were hopefully going to spend anyway, it can be fun to find new recipes and try them. And if you plan correctly and buy in bulk, you can save money this way.

Contribute to open street maps, this is free and helps the community.

Of course the more fun hobbies, and more traditional hobbies do cost money, and can get very expensive very quickly so I do understand your frustration, but finding free cheap hobbies was key to getting out of my depression and I urge you to find something, these suggestions may not be right for you but it just shows there are options