I've been doing repairs on cars since I was a teenager, so the cost of parts on my Challenger hasn't been way out of line from what I expected. The work that absolutely trashed that moving target I jokingly refer to as "my budget" was getting the car stripped and repainted - it turned out great, but it was a huge chunk of $ all at once and well past 2x what I originally thought I would spend. I'm suffering from "mission creep" like many others in terms of looking at one "simple" thing to fix which leads to 3 other things that need to be dealt with: bad wires, chunks of rust in the gas tank, dry-rotted rubber somethings. With the weather still being crappy (snow again this morning!), there's too much time to surf the web looking at potential purchases that would be nice but aren't really necessary to get the car back on the road and have it run reliably. It's still worth keeping a mental list of these items as considerations for future upgrades, but I just keep telling myself to only buy something if it's absolutely needed for the car right now. I'm pretty sure my car will never be worth what I'll end up spending on it, so this "look but don't buy" approach is probably the best way to keep some balance between finding enjoyment out of working on the car without it becoming a complete money pit.
I do appreciate hearing the other perspectives shared on this thread. There's some level of twisted comfort that comes from knowing other people have to juggle some of the same issues I do. Within the last week or two, there was a response to a newbie's first post which includes the words "welcome to the asylum" - yup, that's on target. That said, there are certainly many worse things we could be doing with the time and money we spend on the cars.