It depends on what engine you have. A red battery light does equate to early
signs of alternator trouble. I've also noticed that on many BMW's I've seen
as they start to fail they make a high pitch sound. Mine went on my X5 like
two months after I noticed the sound.
It may not happen to all failing alternators. Labor and all was right around
$1200 about four years ago on an M62 4.4i. So expect the same with a V8
and slightly less with the M54. The radiator fan has to be removed on the
eight's, I don't think so on the six's.