The cheapest grade of fuel available at most gas stations is rated 87 octane, usually designated as "regular" gasoline. Stations that carry the ethanol-gasoline blend E85 may price it lower than regular gasoline, but this fuel does not work in automobiles that are not designed to use it.
Most gas stations carry three grades of gasoline, and while octane ratings may vary, they are usually 87 for regular, 89 for mid-grade and 90 or 91 for premium gasoline. This rating merely measures how the fuel combusts, and premium gasoline is not necessarily a better choice for automobiles not designed to take advantage of the higher octane rating. The owner's manual of a car details which grade of gas should be used, and most cars run on regular gasoline without issue.
Diesel fuel is refined separately from traditional gasoline, and its price can fluctuate independently of gas prices. Before 2004, diesel fuel was often the cheapest fuel available at a gas station, but the demand for home heating oil pushed the prices for diesel up since the two fuels come from the same grade of oil.
While gas stations typically have three grades of gas available at the pump, many only have two underground reservoirs, one for regular and one for premium. When a customer selects the mid-grade option, the pump draws from both tanks, mixing the two fuels to produce the mid-octane blend.