The product cost wouldn't be a big deal (depending on thickness, too), but you'll have to ask the contractor to tally it all up for labor, etc, since it'll take a little more time. (Nothing against them in particular, but don't let someone tell you that "you don't need it" - a convenient thing to say when someone would rather not do it that way.)

An alternative, which has recently been accepted in my jurisdiction, but longer in others, is MemBrain, a "smart" vapor retarder (Class II), which actually changes permeability depending on the amount of vapor in the wall.

Building Science Corp. mentions its use in abovegrade walls, but I can't find mention of it in below grade applications, though MemBrain says tests at the University of Minnesota show it works well in basements in cold climates.

Here is a link that might be useful: The Smart Vapor Retarder

Thanks guys. Do I need to apply Drylock prior to glueing XPS on concrete?