The wholesale price for urea is about $400 per metric ton, after peaking in 2012 at about $700. That's about 2200 pounds, or about 300 gallons. (I am not sure about the weight of a gallon of urea, but estimate it's about 7 pounds.)
So at the wholesale level, about $1.33-2.00 a gallon. Figure a typical markup, and it shouldn't cost over $5/gallon at retail and usually less. Compare this to the price of a gallon of windshield washer fluid, about $2-4/gallon.