I like Wallaby's idea. What did when I worked and rode in jeans (I wore chaps over the jeans, but a 4 -6" strip in the crotch above where my chaps fit always wore out first) was cut up an older pair of jeans and sew a 3" wide reinforcement directly over the area that I knew was going to wear.
I never wore the fancy "riding jeans" or demin breeches, so I don't know how those wear. Nowadays, since I don't work outdoors and only ride a couple of times a week (and since my knees are stiff) I wear breeches, period. I've found the the Miller's Tuff Riders wear like iron, and that's all I wear these days.