Conventional side mirrors account for 0.03 of a vehicle's drag coefficient, according to a comment made by one of the engineers on GM's PNGV Precept.
That figure is repeated & quantified at 3-6% of total vehicle drag for a modern car in a report called "
Contribution of different devices to the total drag " (PDF). (link is now dead.)
I replaced my Firefly's passenger mirror with a convex inside mirror placed just inside, next to the window glass. I figured removing one mirror was good for 1.5% increase in fuel economy on the Metro/Firefly.
Reflections on side mirrors: testing drag vs. MPG - MetroMPG.com
Like many individual mods, it's too small an improvement to detect outside of normal tank-to-tank variability, but it's real.