Please also note that wind speed is usually measured 10 meters above the ground. Wind speed drops significantly as you approach the ground with speed being 0 at the ground (boundary layers). As a rough approximation, wind speed is 1/2 that 1m above the ground than it is at 10m and 1m is where you, the dragiest part of the system reside. Essentially, 15mph quoted does not equal 15mph across your whole bike, if any of it.
Furthermore, it's rare that the wind hits you directly from the front. I'm slowly working my way through discussions from a chap called Hambini who reckons that yaw is much greater than previously suspected as he takes transient airflow into account instead of steady state that most people use.
With all this info, the equation of choice is D = Cd * A * 0.5 * r * V^2. Assuming air density, frontal area and coefficient of drag all stay the same, and if we assume that the wind is only half that measured at 10m (ie. 7.5mph), total velocity is increased by 50% to give a drag increase of 2.25x vs 4x at 30mph. Drag is directly proportional to required power output
So in summary, riding at 15mph into a "15mph" headwind will approximate to riding at 22.5mph.
*there's talk that a still day isn't a still day though, but let's not get into that....