Good point, effective octane mixing E98 with 91 or 93 octane gas to make E50 probably even higher than using 91 and E85.
So if I start with 5 gallons of 93 octane E10 in the tank, I will have 0.5 gallons ethanol in the tank. I then dump my 5 gallon can of E98 into the tank. I now have a total of 10 gallons in the tank comprising about 5.5 gallons ethanol and 4.5 gallons 93 octane, which is E55. Total cost is (5 gallons of 93 octane x $3.35 / gal) + ($40 per 5 gal can E98) = $56.75, or about $5.675 per gallon E55.