The basic price of oil for the next 50 years will be about $30 a barrel. Some of the time it will be higher, but I would bet that a lot of the time it will be lower. The key point is that any investments made in oil or oil substitutes that depend on oil prices staying well above $30 a barrel stand a good chance of losing money. They are imprudent, risky investments--although nobody can say for sure that they won't pay off.
One thing I would like to point out about Mr. Singer's prediction however. He is referring to the inflation adjusted price. In other words, a 50% devaluation of the dollar during that time would result in an inflation adjusted price of $45/barrel. And that is one of the main reasons we always maintain investments in commodities. We fear the Federal Reserve's corrosive effect on the value of the dollar much more than we fear running out of oil or any other commodity. Dollar devaluations - or inflation since it's the same thing- show up in the price of commodities much sooner than the general price level.