What should i ask for at the mechanic shop?

I have a 2003 expedition that just turned 100,000 miles. The check engine light came on and Auto Zone said there is a cylinder 5 mis fire (its feels like its been missing and the fuel mileage has plummeted). I've been told its probably the spark plugs, but i'm bringing it in to the Ford dealership to get checked on Thursday. My question is, Since its just made the century mark in miles, what else should i have done? And what price should i be expecting to pay, particularly for spark plug change? I'd like to get her taken care of because I don't want another car note for a while......Thanks