I've been down this road a few times, and similar to what others have mentioned, it's a matter of what enjoyment you will get versus the cost. There's a satisfaction that comes with the cleaning and wrenching, the aggravation of finding parts in some cases, and almost always, the rewarding final result after your work is complete. If it's a vehicle I know I can't easily replace as a whole, I'm more likely to not feel any remorse when it gets expensive. I feel much less guilt spending the money on parts than I would tearing open a scarce NIB kit, so I'm usually hunting for parts trees and clean donor vehicles. Most of the time I end up with a fair amount invested but satisfied with the completed vehicle for the purpose it was resurrected for. If you know what your expectations are from the start and you have an idea what you're willing to spend, you're less likely to not finish it or be disappointed when it's done.