$8.59 per gallon of gas - is this math right?

Or one of the assumptions here is wrong.

The big missing factor I see is the amount, and selling price, of other products that come out of that barrel of oil. You buy the barrel, refine out the gasoline and sell that... but the leftovers aren't worthless.

From http://www.quoteoil.com/oil-barrel.html :

The largest share of the 42 gallons of crude oil contained in a standard barrel ends up as finished motor gasoline. Motor gasoline accounts for 19.65 gallons of the finished product. Next in line is distillate fuel oil at 10.03 gallons.

Jet fuel comes in a rather distant third place in the refinement process. Of the original 42 gallons of crude oil in a barrel, that portion which will end up as jet fuel is 4.07 gallons. Residual fuel oil accounts for 1.72 gallons of the overall refined product.

Then there's a list of another thirty things that you get from the remaining leftovers.

Rough rule of thumb: add up the number of gallons of non-gasoline products in the paragraphs above and you get about 16 gallons. That's nearly equal to the amount of gas you get from the barrel. Assume the value per gallon is about the same for every product and you get gas selling for about half of $8.59, or $4.30 per gallon, which looks suspiciously like what we are paying right now. Puzzle solved.