Cost is entirely dependant on how you choose to do it Tony.
I'd suggest you sit down with a cup of tea this evening and have a good read through that thread. it gives details of all the various ways people have installed FMIC's on their A3's and S3's, and the good and bad sides of the different installs.
many of us have also written out parts lists and prices, with links to suppliers all in that thread above. there is a lot of info in there.