The two gas engines I have are the "older" ones, a 2003 Durango and a '99 Honda Goldwing. I've done several calculations on each vehicle at random intervals and the results, regarding costs, are pretty consistent: if the cost/gal is <$.10 (E10 is usually less expensive) then I use the regular fuel; if >$.10 then I use the E10. This applies only to the cost/mile for fuel.
We've found the pricing in other states varies greatly. Probably because Iowa strongly supports the ethanol industry. Just because I live here half the year doesn't mean I support everything it does. ;) Hope this helps.