Have you looked at other prices or just the price a this one "local"? This sort of work is typically very specialized and it does sound like a custom job, expect prices and installer "talent" to vary widely.
Screwing up the wiring or ECM of a '08 X5 would be really unfortunate and could be extremely costly to repair. As a consequence, part of what you'll be paying for is the assurance that the job will be done right the first time.
If anything, the $900 sounds very reasonable, IF the installer has a reputation for excellent work.
Edited: My comments reflect my general impressions of aftermarket services and are not an endorsement of this modification. If your X5 has a remote key (as suggested in this Bimmerfest thread
http://www.bimmerfest.com/forums/showthread.php?t=189467), this project would be of high complexity and could potentially undermine important security systems in your car. I consider this modification undesirable from the standpoint of complexity.
Edited Oct-24 by Cale_Johnson