A quart of oil costs whatever the market for quarts of oil will bear.
Because of the way they rationalize purchase decisions, consumers are willing to pay more for something which is rarely purchased.
Consumers are simply not that sensitive to paying a lot for more for a given quantity of motor oil compared to the same quantity of gasoline.
This makes sense: how much do you spend, in total, on motor oil every year compared to gasoline? If motor oil were to increase 50% in price, how would it affect your bottom line compared to a similar increase in the price of gasoline?
So of course you get ripped off at the retail level, where you are buying dinky quantities like quarts and gallons.
The oil change shops that buy the 55 gallon drums also get similarly gouged, because their suppliers know that these shops are running a rip-off business, and they rightfully want a piece of that consumer- fleecing action. If the suppliers charged the lube businesses less for oil, the oil changers would still charge some $20 to $30 for an oil change, and just pocket more profit. The average driver who either doesn't know how to change his own oil, nor wants to expects to pay about that much.
So you see, they get you whether or not you're a do-it-yourselfer.