Umm... both cars will have
Umm... both cars will have that. It's a TRANSMISSION maintenance, not a "diesel" maintenance requirement. Your'e thinking of the after-treatment fluid, which is more like $40. The "D" in DSG stands for DIRECT shift gearbox, not "diesel." So far, those trannies have only been used in VW Group diesel vehicles, but that is changing as seen here.
And if you're paying $850, you're getting ripped off. It's a job any good mechanic can do in about 2 hours for $150 worth of parts. If your garage charges $850 for that, you should find a new mechanic.