The difference in fuel consumption in your example comes from both increased aerodynamic drag from increased speed, AND higher RPM. You can't easily separate the two to figure out the RPM hit alone.
However, for a given constant speed, lower RPM is better.
Related:
Showdown: testing RPM vs. MPG at a fixed speed - MetroMPG.com
Speed kills: testing MPH vs. MPG in top gear - MetroMPG.com