I've found some pants that I use for cool weather riding and they are very inexpensive. They are considered base layer pants but work great for cycling.

If you're interested, go to ebay and search for "paradox pants". I'm a size large, but I buy these pants in men's medium so they fit like tights. They are not real heavy and are very comfortable. For extreme cold you'd want something over them, but I've used them down to 40 degrees. I've bought them for as little as $12 shipped. You can't go wrong really.