I own a 5.4 too, one dealership wants around $500 and if the break them the fix is included. I've been told that dealerships typically charge an additional $150-$200 for each break-off. I don't know what I'll do yet, I typically change my own but no experience in removing the busted ones kinda spooked me.
I think the tool is called the Lisle tool? I can't remember but I know you have to be very careful not to get any debris like the crumbling porcelain from the broken plug down in the cylinder. I'm generally a DIY guy but with all the problems that these motors have it's best to hire this one out IMO.