The reason oil prices have come down is because OPEC has cut prices arbitrarily. They were artificially high to begin with. On average it costs an OPEC nation ~$30 to produce a barrel of crude. They're still making a shitload of profit.
It's not arbitrary at all, and it's not really OPEC, as many OPEC members would rather see higher prices for their own internal political reasons. The primary driver is Saudi Arabia, who is trying to crush the shale oil producers in the US (particularly, and to some extent Canada) where oil cannot be produced at profit for less than $80 a barrel. A sustained price below that point is intended to chase US firms out of the market, as they can't produce profits at this level, which in the long-run would be good for Saudi Arabia.