So the gas light came on at 280 miles on my all gasoline tank, not really any different that my high ethanol tanks. Weird. I did take an on-ramp quite aggressively when it came on, so maybe it would have done more under normal circumstances.
Anyway, the topic at hand:
gallons e85: 4.511 @ $1.649/ gallon
gallons of 87 octane gasoline: 8.161 @ $1.949/ gallon
ethanol content of tank: 36.64%
Octane of tank: 93.095
savings per tank compared to 93 octane @ $2.459/gallon: $7.82
No CEL so far.