I usually factor a 60-70 cent spread as being the break even. I haven't used E85 is quite a while, but when fuel was pegging $4 a gallon and higher several years ago, my vehicles lived on E85 for almost 2 years as the price difference was anywhere from $1 to $1.50 a gallon lower for E85. So even with the lower mpg, I was still ahead when cost per mile is factored. I do use E15 frequently. The price is typically a dime lower per gallon than E10 in my area, and I haven't seen any appreciable mpg difference.
There are a lot of horror stories about ethanol generating more wear and such in engines. While purely anecdotal, I find it hard to believe. My 1974 Pontiac Catalina with a carbureted 400 in it, it got E10 from the late seventies until we scrapped the car in the mid 90's with over 250,000 miles on it. The engine was still good. The car died of body cancer. Never had to work on the carb or fuel lines. I have never seen first hand or experienced a ethanol related issue with any vehicle. Now, true, I have seen some smaller OPE problems. But that seems to be more related to letting fuel sit for extended periods in tanks that are vented and a lot of condensation builds up in them and bonds with the ethanol. More about user error than the fuel.