It cost me $3.75 for a Tall.
There's a price premium over Starbucks' Latte or Cappuccino (which cost $2.95 for a Tall in my area) due to the double shot of espresso in the Flat White (you don't get a double shot in a standard Latte or Cappuccino at Starbucks unless you order a Grande or Venti).
Compared to a Starbucks Latte, the Flat White is less milky and there's a solid coffee taste to it. I think they must have toned down their coffee bean roasting as the espresso didn't have the harsh burnt taste I remember from getting a short Cappuccino from Starbucks many, many years ago, way before artisan coffee shops were a thing (which is what Starbucks is aiming for with the new drink).
So yeah, the coffee's actually pretty decent. A bit one note, but smooth. The milk isn't the promised velvety consistency though (it's supposed to look more like this). It was mostly the same dish soap-type bubbles you get with their standard latte and doesn't have the sweetness of micro-foam that's pretty standard for a flat white.
Overall, Starbuck's Flat White was actually pretty good on the coffee side but disappointing as far as the milk goes. I feel like a lot of people go to Starbucks for flavored lattes or Frappuccinos, but the Flat White is worth a try, if you actually want to taste the coffee.
Nutritional Info - Starbucks Flat White w/ Whole Milk - Tall (12 fl oz)
Calories - 170 (from Fat - 80)
Fat - 9g (Saturated Fat - 5g)
Sodium - 115mg
Carbs - 14g (Sugar - 13g)
Protein - 9g
Caffeine - 90mg