Cylinder doesn't fire regularly - what is repair? and how much to fix?

First and foremost, run a compression test on that cylinder, that is step #1, it needs to have at least 150 psi compression. Then, just because the plugs and wires 'look good' is no guarantee. I have seen brand new cars in dealerships have a bad spark plug with 10 miles on it, it can happen. Also, these 4.0 Ford V6 engines get vacuum leaks in the manifolds if it is a plastic manifold, so is there a check engine light on and if so what is the fuel trim, that will tell if the misfire is vacuum leak related or if it is a wire, coil pack, plug ( your engine has no distributor, it has a coil pack ) as long as the compression is 150+. If you have any P0200 type of P0201, P0202 etc codes then that is a bad injector, which can also cause a misfire.