Crude oil, and by extension, gasoline, is a non-renewable resource; therefore eventually, the cost of gasoline will inevitably rise due to scarcity. A vast number of factors play into the cost of gasoline. Experts can only speculate whether gas prices will rise or fall.
Gasoline is a processed form of crude oil, the production of which is complicated. Drilling, politics, barrel import, refinement and taxation all play as variables to the eventual price that the consumer pays. Current environmental trends are trying to move away from the use of fossil fuels, driving the demand for oil down, and leading to lower prices. However, increased scarcity and compiled drilling costs threaten to increase the price. Political standing from oil-exporting nations is always wavering, leading to price fluctuations.
Eventually, the amount of oil available will reach an unsustainable point. The laws of economics dictate that scarcity breeds higher product demand, which is met with a higher price.