BMW Repair: BMW 325ci 03 Alternator problem, bmw 325ci, hard job

Question
I have a Bmw 325ci, 2003.  I just changed the battery today because the car was having starting problems recently.  After installing the battery, the guy told me that when I revv, the meter on the alternator checker (pardon my ignorance of auto language), is at 12 when it should be between 13 and 14.  He told me I need to change it and it'll cost about 400+ to replace the alternator else my newly installed batt will die down.  I just spent 200 on the batt!  Is there any other alternative?  Please help.

Answer
Yes the voltage reading was low.Although most place would replace the alternator,the chances are if you just replace the regulator/brush assembly in the alternator all should be fine.The problem is the brushes are worn.Taking the regulator out isn't a hard job and it is a fraction of the cost.