not expensive, just a bit of work
I had the speakers already, from a prior car, but they aren't all that expensive, maybe 50 bucks. It's not too difficult to do, either, if you're handy at all. there are 10 screws that hold the back panel of the liftgate speakers in place, take those out, take the panel off, and there are 2 screws holding each of the speakers in place. Once you have them off and disconnected, you'll need to cut out the existing circular speaker mounts, drill 4 new holes for the new speaker bolts, 4 nuts and bolts later and you have a significant upgrade in sound from the back.