Try doing a search. This question gets asked regularly. The kind of responses you can expect to find/get: $250-$450, depends on who does it and where you bought the headers from. You are better off doing it yourself. It's not that hard and you can buy all that you will need to do it for less than you would pay to have it done. That way you end up saving money and gaining tools and experience.
PS: Do not expect to do it in four hours. If you have never done anything like that before and you take your carefull time, it can take awhile. I do'nt want to say how long it took me, but it was no five hour job!