As far as Firestone... it all depends. I have a friend who is a mechanic at Firestone, one of the few good ones. He's looking at openning his own shop someday. In fact, any mechanic place will entirely depend on the quality of their employees. Quick tire places don't care much, therefore most will suck for anything beyond installing tires and doing oil changes.
As far as quotes for a job, they are typically generic calculations of time and parts. On some cars, changing plugs is very difficult. On the P5, it's a piece of cake with frosting. You could probly haggle with them to get the price down... or just do it yourself.
Also, it's a good idea to learn how to do some of this stuff yourself. The air filter is very easy and might cost $5-$10. The plugs are easy and cost a buck or two a piece. I haven't done lightbulbs yet, so those might be a little more tricky. I've always found it far more valuable to spend the time to learn how to do it. It might take me a day to do something that a professional could do in an hour, but I'm generally far wiser once I figure it out on my own.
