Well, besides the price, the 10-dollar capes are made in china from PVC, a material that is not meant to last forever, or even all year. So for ten bucks you can stay dry for a few weeks, and then add to the landfill. Or you can spend the extra money on a cape that is made of fabric (rather than PVC), seam-seal it and mend it as needed, and expect it to last you a decade or more.
Carradice makes a great cape (their Pro-Route model) for about 70 bucks retail. I haven't seen the cape from CAT but if it's fabric you can assume it will last longer than that 10-dollar job.