I just got my 100k tune up on my '05 Expedition and was shocked by the cost. $500 to start plus $50/plug that breaks while being removed, which they have a high tendency to do. The dealership broke 2 of the 8 on mine and they have plenty of practice. I figure changing them earlier could reduce the chance of them breaking but the plugs alone are $20 a pop so that pretty much wipes out any savings from changing them sooner.
Overall, the truck has been trouble free but $600 seems pretty steep for what is typically a pretty easy DIY job.
end rant.