At the station by my house that has it, E85 is 3.13 where 93 is or was 3.79. I remember reading somewhere about a guy tunning his 6.2 on E85 and was maintaining his mileage in the mid 16's. (but cant remember who or which forum i read it on)
So if you were to tune specifically for it and pull down mid 16's it would easily be worth it. Everyone that runs it seems to love how their truck perform with it.
Plus comparing it to 87 there isnt much of a cost difference but compaing it to 93 there is a pretty substantial difference.