The average cost to repair the alternator on a BMW 325i is $396 to $621, as of 2015, according to RepairPal.com. This is almost the same as the average cost to repair the alternator on any car, which ranges from $402 to $640.
When taking a vehicle in for an alternator repair, it is best practice to check the entire system that starts and charges the vehicle before replacing the alternator. Other problems may be causing the alternator's performance to fail. For example, a battery that is weak may negatively affect the alternator. It is also best practice to change the alternator belt at the same time an alternator is repaired.