I have visions of building such a range when I become a landowner. I would eventually probably like to incorporate electric motors and use several different programs to design custom movement patterns for targets.
The simplest and cheapest I am currently considering would involve four pulleys attached to stout posts in a square. A rope would run around all these pulleys with reasonable tension, and a range buddy could stand behind the shooter and move targets hanging from the rope, by simply grabbing the rope and moving it back and forth. This could easily be very realistic practice, but someone has to help you (and you could help them in return).
Of course you might shoot the rope, but its only a few dollars for a 100' cheap rope at Harbor Freight and a cable would be even more expensive, and almost as vulnerable. This whole setup could be built in an afternoon for not much money.