The rear plugs aren't all THAT hard to change. I've changed mine on my 98 V6 Mystique many times. I know some people say you don't need to remove the coil pack but there are only four 7mm bolts holding it on and it's easy to do. I've never removed the intake manifold to change the plugs.
After I have the coil pack out of the way I twist the plug wires back and forth rather than pulling straight up on them so I don't damage them. After I have the wires off the plugs I use a 3/8 ratchet with a couple of extensions and a universal swivel along with a spark plug socket. The little beggers are really not that hard to remove and I certainly wouldn't pay anyone $500
to change them for me. Man, for that much I could really buy a nice set of tools to do just about everything on my car.
Pay close attention to what wire goes where when you're removing them, be sure and reattach the two ground wires to the coil pack when you put it back on and use anti-seize on the spark plug threads.
There is a How-to on the CEG website that tells step-by-step how to change the plugs, including gap setting and torque specs. Click here for a shortcut to it:
Duratec V6 Spark Plug Changing