I did DIY on my 2001 4.4i. It is a little time consuming but nothing very hard. Painful to have to drain the coolant to remove the alternator (on the 4.4 at least, the pictures of the 4.8 alternator look like an air cooled unit)

From the 2006 4.8 in your signature, the dealer part is this one - note that they are remanufactured, not brand new, even from the dealer:
Part Number 12317540994
Part Name RMFD GENERATOR
MSRP $670.27
Core $50.00

The dealers that discount online sell the part for quite a bit less - I saw it for $442 at Morristown BMW.

You can find a Valeo unit from places like eEuro parts for $280 or so.

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David.X5
2001 X5 4.4i Sport
SOLD! at 160k miles