How much is the door listed for in the store? Subtract that from the overall quote, throw a few percentage points in for taxes and you will have a rough idea.
I have had Lowe's install doors. They provide you a set price. If the contractor can't get the shit done in a timely manner, the contractor eats it. The cost isn't passed on to you. Also, the contractor actually shows up because he doesn't want to piss off Lowe's. I doubt you will get a better deal hiring a contractor on your own and buying the door separately.
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http://bible.cc/1_corinthians/13-11.htm