Seattle BMW wanted 600 for parts and labor to install the mounts and fit the top on my car. I decided to pass on that do it myself. It's actually not that hard. I'd say it would take 1-2 hours to figure out the first side, then about half that for the other once you know what your doing. If you're comfortable with the thought of removing the radio or any trim pieces this isn't that much harder. You do need a good collection of torx bits though.
The most difficult part is removing the bolt that holds the bracket to the convertible top assembly. ( The black arrow in this picture
http://i51.photobucket.com/albums/f3...iver700001.jpg the picture is a cutaway ) On my car part of the body was blocking the bottom of the bolt on both sides. It looks like the proper way to do it is to remove/loosen the top assembly so you can get the quarter inch of clearance you need. I just bent/cut the metal that was blocking the bolt, then bent it back when I was done.