How much a barrel of oil costs is due to a number of factors, for instance, what the demand is, how much oil is available on the market, the production costs, the value of the US dollar and of course, the speculators. Oil is bought and sold in US dollars, so the value of the currency is important to the cost per barrel. There is only so much in the way of oil reserves and as these become depleted, so the price will go up. The continued industrialisation of large populous countries such India and China has also led to increased demand thus leading to higher demand.
Oil supply is controlled by the countries that export it. Opec meets regularly to set the amount of oil that they are going to release onto the market. Approximately 35m of the 80m barrels of crude released onto the global market every day comes from Opec. This means that they can control the oil price to a degree by increasing or reducing output.