I definitely agree with the "not worth having special purpose stuff" sentiment. I think I just own so few pairs of pants with intact crotches that "old clothes" aren't an option.
Since I'm not legit enough for crimpy or Rickster's suggestions (never mind the drivers at the hairpin turn, I feel sorry for the belayer!), I'll see about a pair of Carhartts. My girlfriend said she had to break hers in using a cement mixer full of stones, but I suppose that's not too dissimilar to what climbing will do to them.