I assume its $500 for just the parts. This OEM BMW ULF box is just stupid money for the e46. I can't see why it would be cheaper for the e90.
The electrical connections are pretty straight forward. You unplug your HU & insert this connect2 (or quickconnectproducts) adapter kind of like a short extention cord & then plug that into your HU.
The most intensive removing your dash peices to remove your HU and routing the mic wire up the A-pillar to your OEM mic position.
Last note - I have only done this on my e46. I just assume the connect2 setup will be similiar for the e90.