A friend noticed that since he first started driving (about 30 years ago) that the price between octanes or grades of gas hasn’t varied much. Why is that? Why hasn’t there been a noticeable spread in price between fuel grades?
TIA 
My WAG would be that while the base price for oil and gasoline varies significantly over the years, but the cost of refining that gasoline to produce different octane ratings is relatively stable since the technology more or less remains the same. That would explain that while the base price for gasoline varies but the price difference in various octane ratings remains roughly the same… Anyone with some actual knowledge care to chime in?
Because the higher-octane fuels are simply the lower-octane fuels that have gone through some additional processing steps. The cost of the additional processing does not change: approximately $0.20 / gallon for 93 octane.
Higher octane fuel explodes less easily than low octane fuel.
Diesel explodes a lot less easily than both - yet is cheaper
My WAG is that the cost of production of high octane is no different than that of low octane, or possibly that it is actually lower since it is more frequently required.
Diesel is often (always?) more expensive than gasoline, at least in the US. Wasn’t always that way, but it does have more BTU, and is a less refined product. Marketing forces removed that little inequity, I’m sure.