It's tough. In the long run, it's always cheaper to maintain a vehicle well. But if you have a car that hasn't been maintained, or that just coincidentally has everything going wrong with it all at once, it sure doesn't seem like it.
It's kind of a pay me now or pay me later kind of thing.
The other posters are right- you can change every mechanical part in the whole car. But if the body starts to go, it's a goner. Or at least, you are talking restoration, not just repair.