It is typically between 1.5 and 3% of fuel usage (1.5 to 3 gallons of urea to every 100 gallons of diesel). The Urea tank on the Dodges have left me a little wanting since they're only 5.5 gallons in total. Personally, I bought a 2.5 gallon container of DEF that I re-use by filling at the truck stops. Cost per gallon at stores such as Walmart seem to be around 5 bucks a gallon give or take 2 dollars. At the truck pumps, seems to be anywhere between 2.5 and 3 dollars a gallon. So in essence, it adds about 5 to 10 cents per gallon diesel depending on whether you're buying in bulk or purchasing in a container. There is a shelf life, but it mainly diminishes in the summer to around 6 months at 90 something degrees. But typically, you can get a year or two for shelf life. I've been entertaining getting a second container to haul DEF around with, but really haven't had a need as of yet.