I think part of the reason for the high octane numbers is a difference in measurement, but I could be mistaken.
Anyway, in Yuma, Arizona, regular unleaded (probably 87 or 89 octane, never looked) is $2.37 and diesel (unknown cetane) is $2.41.
Cetane in the US is usually in the range of 40-44, and some people are lucky enough to get cetane ratings betwen 45-50. If you can find biodiesel in your area, that has higher cetane of course.
I've always found that my car gets better mileage with the rot-gut, low cetane fuel, but it sounds like crap and smokes(comparatively speaking). I understand the noise/smoke, but not sure why the mileage is better. Maybe just coincidence. Diesel fuel for most of America sucks, and it's practically a necessity to use a fuel additive(I always do) for raising cetane, adding lubricity, and dealing with water.