Yeah, I think it's easy to start spending a lot of money on something once you get into it.
When I built my TMNT cab, I thought it wouldn't be too much (since I already had a CRT and the game PCB.) Just some wood and controls, right? Even without the replacement CRT I had to buy, it still came to around $1150!
Below my profile pic, you might notice it says "Robotics". Who knows how much I've invested in my robot hobby over the years. I have lots of robots, from a Hero 1 to a Roomba 880. Lots of kits I've built too (I've got a hexapod one I still need to fix...). And I have boxes of spare parts (some of which get used in my arcade stuff, so it works out some.)
Another hobby I have is freshwater aquariums. I have a 29 gallon tank with live plants, dwarf crayfish, cardinal tetras, plattys. Did a major overhaul on it a few months ago, and spent some good coin on new plant substrate (the dirt in the bottom), new plants, a better filter, upgraded to new hood and LED light, aquarium plant fertilizer, and more. Plus, I'm planning on getting a $300 pressurized CO2 system for my tank (mixes a small amount of CO2 into the water. It's good for the plants [photosynthesis]. In fact, some aquatic plants absolutely need CO2 in the water.)
Finally, I recently started getting into drones (quadcopters). I have a nice Zerotech Dobby: a simple drone I control from my phone for only $350 (hey, that is cheap for a drone!) Part of me, though, wants to run out and get a DJI Mavic Pro drone, which goes for $999 new (check the reviews, its a great drone.) I'm trying to resist, though. (not because of cost. I can afford it. But I live within 5 miles of a Navy jet base, and there are strict rules about flying these. The Dobby I can fly in my house or back lawn. The Mavic Pro...I'd want to go fly it out in a huge open field without worry.)
Anyway, the point is, all hobbies can get expensive. There are ways to cut costs, but you'll end up spending that money on another part of that project. I bet even stamp collectors have spent too much money on their stuff.