I have some $25 rain pants that are too short. I got some raincoat fabric at the fabric store and sewed extensions on the legs. Then, because I had punctured the pants with the needle, I had to seal up the seam with yellow duct tape (to match the yellow pants). Not pretty, let me tell you, but it works.

I like the gaiter idea better. If you put them UNDER the pants as suggested, the rain will run down the pants and onto the gaiters, then down the gaiters to the tops of your shoes, and then off your shoes or into your shoes depending on what kind of shoes you have.