I think this is quite regular phenomenon we are facing.
My weekend example: valve rocker at MB - EUR 130, Febi rocker, exactly same, EUR 20.
I have 280SL where you cannot buy a U-joint separately at MB, yet we go to a specialists who just replace U-joints as they are universal. I suppose the U-joint you are looking for is pretty typical U-joint too. Probably it is about finding part of good quaity. Maybe it is better to take the car to the shop who can replace the joint and balance the shaft. They will give quarantee for repair.
On the other hand: Daimler are keeping a huge stock of parts for mobility guarantee (at least they try, but it is not 100%). The more unique parts, the more costly it is to keep the stock. It must cost money.