I just found this. It's been posted here before and comes from a test in a German cycling magazine.
Needed Watts for Speed = 45 km/h :
Stevens San Remo bike with normal handlebar 465 Watts needed to go 45 km/h
Same bike Hands down the drops: 406 watts needed
Same bikeEaston Aeroforce bar: 369 Watts
Same bike Triathlon position (5.5 cm lower bar, saddle forwards): 360 Watts
Same bike Triathlon position (5.5 cm lower bar, saddle forwards) and
carbon Tri spoke wheels front and rear: 345 Watt
So you gain 37 watts with aero bars as opposed to the drops