well....16 gallons of 91 would be roughly $ 70, e85 would be 48, so savings of $22...that's about 30% saving. BUT then you have the MPG factor.
It's just not worth it in your particular case. E85 in general isn't worth the retune and sometimes required additional fuel mods for a bolt-on car. I guarantee you'll spend that $22 in savings on at least another partial tank of E just to finish your normal workweek.