I got a grande for $4.95.
Unfortunately, my local Starbucks had run out of the sour blue powder by the second day of the promotional event (and apparently, supply--and frazzled Starbucks baristas--are an issue at many-a-store). I originally thought they had made my drink wrong when they gave me the purple drink with nary a blue speck or blue drizzle in sight.
I really looked forward to being able to see the colors change with mixing, but I only got half of the Unicorn Frappuccino experience and I didn't like the flavor enough to try a different location. If the mango syrup was stronger in actual fruit-flavor, I might have considered it. The purple color of the drink really was lovely though.
Nutritional Info - Starbucks Unicorn Frappuccino - Grande with Whipped Cream (16 fl oz)
Calories - 410 (from Fat - 140)
Fat - 16g (Saturated Fat - 10g)
Sodium - 230mg
Carbs - 62g (Sugar - 59g)
Protein - 5g