I bought regular nylon pants from Cabela's:
http://www.cabelas.com/cabelas/en/te...form23&Go.x=32
They used to be $12 on sale. They are very thin and waterproof, though I haven't done more than 25 mins in the rain with them. Other reviewers say they hold up very well for a day trip or so.
I hear Gore-Tex is awesome, but I hesitate to pay the 10x-15x price.