I agree on dealerships being outrageous on diagnostics. My local dealer charges $95 and almost always just goes by the codes that were thrown. $95 for 2 minutes is robbery. When I go there because I just don't want the hassle of doing the work (I just let them replace a water pump in 4 hours) I go in wearing the hat I got from a used car dealer and tell them what I want. They are very pleasant and seem to know I won't fall for the framus oil needing flushing. Then I go sit in the waiting area and listen to the old ladies telling me how framus oil should need flushing every 6 months on a vehicle this expensive.
This dealer didn't change my plugs so they always recommend it for $225 so it sounds like your diagnostics were expensive. I ALWAYS read the codes (Walmart has a $20 reader that reads most) before going to a dealer even when I know exactly what the problem is.
"It is useless to do with more what can be done with less." William of Occam