Had an informal 25 mile competitive session against some random middle-aged guy on the trail, him wearing a cycling cap, me with a helmet. I caught the guy from maybe 400 yds back over about 6 miles, it stayed competitive for maybe the next 14 after that, and I was able to slowly build a 200 yd lead over the final 5 miles. Before you attack me for mercilessly beating the middle-aged guy, I am also a middle-aged guy. He had a newer bike, a Cervelo TT with aero bars and tear drop seatpost, vs my older road bars steel bike, so that should have given him an advantage along with the Vroomen and White design, supposedly worth 5 watts against other newer bikes if you believe the marketing hype. Maybe a fitness difference explains my ability to overcome the disadvantage.
My question is, comparing a road helmet to an old school cycling cap, it seems obvious that more cross section area dragging in the wind would have some cost. Neglecting the fact that event sanctioning bodies will exclude riders without helmets, can anyone quantify what it costs in terms of speed or power?