Yeah, I would shop around. I have seen prices vary but for someone who knows what they are doing it shouldnt take as long as someone who has never installed coils on a M3.
Some places go by the book and usually do it faster if they know what they are doing.
To get the coils installed, dialled, and alignment it should cost more than $400 from my experiences but shops do vary in price and maybe someone from the forum could reference you which would be your best bet. I would say $300 is a deal but only from a shop who has done this install many times and not a shop that just needs to make money and will play around to get it done.