Free: Contests & Raffles.
I like the ones w/cargo pockets that I got from Cabela's, $100.
I've had a bunch of military surplus wool pants over the years that worked great. Right now I have a pair that were from the Canadian army. I got these at Ed's surplus in Lynnwood, don't know if the store is even there any more. They were about $20. I can't find them any more but if you come across a pair of the German ones with the double knees, grab them, they are great.