I was wandering around an ACE Hardware the other day and they have a whole section of very affordable bib style pants - cotton/canvas and a waterproof version. Of course, you'll look like you're riding to go work on an oil rig or something but they were under $50 and looked very rugged and durable. Motorcycle specific overpants are available many places but price and quality are highly variable - Dennis Kirk, Ebay, CraigsList or look in the ads in the back of American Rider or similar magazine.