That's OK, DEF is cheap and easily accessible in my book. We spend more going out to eat in one meal than the money I would spend on that in six months. A non factor in my purchase....
It's not entirely a literal cost comparison, but also convenience/time.
At what ratio of gallons of DEF to gallons of diesel would it no longer be acceptable from a convenience/time perspective for you? (ie. if everytime you had to fill up on diesel, you also had to also fill your DEF tank)
I think with emissions becoming more stringent, DEF consumption will probably increase in newer vehicles over time.