In generic terms, a Flex-Fuel vehicle (a vehicle that accepts any mixture of gasoline and ethanol from 0% ethanol/100% gas, all the way up to 85% ethanol/15% gas) has a mileage depreciation that hovers right around 20-25% from standard gasoline.
So, given a car that gets 30MPG on gasoline with a 12 gallon tank, at today's average price of $2.99 / gallon (according to fuel gauge report), would cost you $35.88 for the tank, which yields a range of 360 miles at a final cost of 9.9 cents per mile. Given the hypothetical $1 per gallon ethanol, it would cost you $12 for the tank, and would yield 270-285 miles for the final cost of 4.1 - 4.5 cents per mile (well under half the cost of current gasoline).
In terms of effeciency, ethanol's biggest problem is that the range is significantly lower with today's ICE. If you take a look at projects like the Chevy Volt, and other GM projects, you'll see that they are trying to add a plug-in charging battery / hybrid system on top of their current FFV fleet, making the range that much better / supplemented by battery technologies. If the Chevy Volt lives up to the hype, it's going to have an approx 500 mile range using ethanol and battery.