Rocy Mtn states have octane ranging from 81 to 91 and ethanol mixes ranging form zero to 10%, and now 15%. Prices for the low grade stuff also range form a low of $3.25 in Montana to $3.95 in Colorado.
The rub with putting 15% out there and saying it is only for 2001 and newer cars is that not all the stations are going to have the tanks and pumps to support E10 and E15 in three different grades, plus E85, and diesel, so there are going to be some weird girations going on at some stations, some confusion for other drivers who don't know the difference, and issues with some vehicle damage in time because of the ignorance.
Yes, there are a number of subsidies around enthanol production. I'll admit ignornace to understanding how much and how far they all go. I've never bothered to research it much. Like I said earlier, the US government has surely screwed it all up. Ultimately, DC is such a mess that the whole enthanol issues is a drop in the river of a set of much bigger issues they refuse to deal with.
If you want to know what Washington is doing that directly impacts the car hobby, be it E15, noise, modifications, lighting, etc, consider joining the SEMA Action Network. They publish a monthly newsletter of various activities of Federal and state legislation issues impacting enthusiasts.