Windshields are made by 2 or 3 companies worldwide, and once they slap the OEM logo, the price of said windshield goes up.
I changed the windshield on both the e39 and e53 with "aftermarket", no issues. On both the rainsensor & light sensor work fine (recalibrated at home with GT1/INPA after the install).
The BMW windshields come in 2 basic flavors, with and without the rainsensing/light sensing cutout & attached lens. Then they vary for both styles - comfort, yada yada.
Another reason BMW will charge you more, because you NEED the OEM shims around the windshield, so it sits correct and you have no wind noise at speeds. The shims can be re-used if the glass company knows what it's doing. Otherwise if destroyed you need new ones. One could contact the dealer and ask what's needed to install a new windshield. He'll give you the list, and you can pick everything else except the windshield.

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Stable: e92is, e46 M54B25, e83 N52, e53 N62 - sold, e39 M54B30 R.I.P.