E85 doesn't sell for more than regular gasoline - if it did there wouldn't be any reason to produce it. I just moved out of Illinois (damn I miss Tampa!). Regular gasoline (87 octane) was around $2.84/gallon, whereas E85 was only about $2.40/gallon - that's a $0.44 difference!
Most normal cars cannot run E85 effeciently due to E85's different optimal A/F ratio. Most newer forced-induction cars can run a certain percent E85 safely, however.
I recently conducted a small test on my GTI and father's WRX. I've read that a 50% mix of E85 shouldn't do any harm. I've even read where a WRX owner is running 95% E85 with larger fuel injectors and has lowered his 1/4 miles significantly. I wasn't willing to go quite that far, yet...
Anyway, my GTI was running pretty low on fuel so I decided to give her a test of this corny new stuff (pun intended). I put in $10 of 93 octane from Shell and $5 of E85 - worked out to ~27% E85, which put my tank at ~25% ethanol. No ill effects, car was definitely a little peppier. With only 25% ethanol, the difference wasn't much though.
Shortly thereafter I took a trip to Oklahoma (don't ask) where E85 isn't available - neither is 93 octane! This new GTI hates 91 octane
Anyway, this past Saturday I get down around 1/2 tank with 91 octane in there. Decide to get half a tank of E85...unfortunately I wasn't able to drive the car with E85 in there (I was actually driving a rental truck as we were moving at the time). Over the drive (all highway) my roommated commented a couple times that the car was definitely quicker, but stumbled a bit between 3000 and 4000 rpm. Maybe I had a bit too much E85 for the ECU to compensate for... Fuel mileage went WAY down! The GTI averages around 32mpg highway...with 50% E85 she was getting around 24mpg!
On the other hand, my father's WRX was also filled up with 1/2 tank of E85 (BP or something 93 octane consisted of the other half). The WRX most definitely gained a little oomph, nothing to worship though. Fuel mileage didn't stray too far from normal. Both cars are back to Oklahoma's shtty 91 octane
It really depends on the vehicle...Subaru's ECUs tend to be able to compensate moreso than other manufacturers (like VW, lol!).
BTW, E85 is around 105 octane.