At what point do you trash her?

[crawdad]

This question is probably unanswerable, but I'll ask it any way. At what point do you consider a truck not worth repairing? I know that if you've got enough money and time, anything can be repaired, but considering rust, mechanical problems, sheet metal, etc., when is it wise to stop and try to find something else? Hypothetical question, I know, but I'd sure like to hear what you folks think.