I decided that knowing how many gallons it would take when the light comes on is more worthy information than how far I can go with the light on (since I always carry DEF with me).
So I went to the car wash (so I could rinse off any potential spillage...) and filled it up to the rim.
5 gallons of Def. $28 after tax @ my local autoparts store.
It took ~4 gallons to fill it.
8553miles/4 gallons = 2138 miles/gallon.
$28/5 = $5.60 gallon
Conclusion: The consumption of DEF over 1000 miles costs less than a gallon of diesel.
Average fuel fillup: $80.
Average fuel range: 400 miles
8553 / 400 = 21 tanks of fuel used over 8553 miles.
21 tanks of fuel costs: $1710.60
So for every $1700 I spend on Diesel I spend less than $28 on DEF which is waaaaaay less than I spend on RedBull over the same amount of driving