I took mine to Tune-Up Masters and I was charged 159.99 because of the extra labor they had to do. On Montero Sports they have to remove the air intake to reach the other spark plugs.
About 400.00. They have to pull the upper engine manifold off to get to the rear three plugs, and each plug is platinum, and each has its own coil pack. Used those can be found for 50 ish apiece, or...
Depending on the local shop or dealers... I have a 98 Ford Expedition XLT - 4.6L and I got a quote for $600.00 at a shop... So I decided to due it myseft... It took me almost 4 hours to due...