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The only time I see this seal fail is when the upper intake plenum has been removed and reinstalled. The seal fits around the rigid EGR tube that comes up from below and is often torn during the operation. Otherwise, my experience is that these seals are durable and haven't shown a problem otherwise.
Was there a reason why the intake plenum would have been removed? Like to replace spark plugs?
If spark plugs were replaced, the three on the passenger bank definitely require intake plenum removal. If that's what happened, the dealer is at fault for the new problems you're seeing and you should no be held responsible for more than the cost of a new seal (which should be replaced during the procedure anyway).
is the price of $100.00 reasonable for this service they said this is half price
I would say yes if the part truly failed on its own. Intake plenum R+R (remove-reinstall) will typically take at least an hour to do, which would be consistent with most shop labor rates... about $100/ hour.
But check your service receipt for the possibility of spark plug replacement. There will be six Champion platinum tip spark plugs involved (number escapes me) at a typical dealer cost of about $10 each. If you see spark plugs, the intake was definitely removed and you shouldn't be held liable for any part of the repair.