Go to your local hardware store and purchase a long flexible drill bit. You can use these to drill small holes in the sill/floorboards/whatever inside the wall via a cutout that you'll have to make anyway for a jack plate. It's then pretty easy to run the cable through that hole, as long as you make sure that you cut a hole in the drywall at the right place below it. Given that your house has studs 20 inches on center, at most, you don't have to fish around too much to find the cable once you've threaded it through. Nice and tidy.