Here is a little tip I use and its only because BMW charge so much for each version that I do the following.

Go down to a dealer try not to use the same one every time Take the BMW model you have for a test drive make sure you request one with SAT NAV also make sure you have your laptop with you or drive the car back home or to your office take the DVD/CD out of the nav system burn a copy then put it back and return the car.
This only works if the car has the latest version maps and you get to take the car without the salesman. I know some of you may frown on this. But lets do the Math. To replace the old UBD or refedex depends on what state your from in AUS would cost around $30 - $40 per year. To replace the NAV maps from BMW cost $300+ I would be more than happy even if it cost me $100 a year to but it but $300 a year thats a little rich.