The DEF urea (aka “cow piss” to industry insiders) is about 12 bucks a gallon at WM. Similar to motor oil prices.
On big rigs it is consumed at about 2% rate of the diesel fuel consumed. Not sure about the VW.
The way it was put to me when I looked into it was that they have urea “packs” that have to be replaced periodically, and good luck finding anyone who can do it other than the dealer. So, it’s expensive.