BMW Repair: Battery light & radiator overheated, coolant system, radiator cap

Question
328i 1999, 56000 miles.  I was driving about 60 mph for about an hour and I stepped on the gas to pass someone and my car made a funny noise.  The battery light came on immediately.  The radiator light came on after about a half mile.  I parked it after a mile.  As soon as I turned it off, I think it was the radiator that blew.  Smoke was coming out from under the hood.  Do you have a diagnosis?  How much should I expect to pay?    Thanks.

Answer
Sonja,
 It sounds like the serpantine belt broke.  The serpantine belt turns the waterpump and the alternator.  When this belt brakes it stops turning the waterpump and alternator.  This is why your battery light came on (alternator wasn't turning).  This is also why your radiator light came on shortly after.  The engine was overheating, because the waterpump wasn't pushing coolant into the engine to cool it off.
 The "smoke" coming out from under the hood is actually steam from some gasket/hose that broke on the coolant system.
 There is no way for me to make an accurate diagnosis on what will fix the car and how much it costs.  The area you live in will affect labor rates (i.e. New York vs. South Dakota).
 I hope that it's just a hose that broke, or maybe just the radiator cap released pressure like its supposed to.  If the engine didn't overheat long then it's possible to get out with just replacing the serpantine belt (pretty easy).
 Good luck,
 Josh