How much does fuel cost for a plane?

This question is best answered with a "Depends On" type of answer.

There are several types of Aircraft that use several types of fuel.

There are common piston type aircraft that use anything from common no lead gasoline to jet aircraft that use a higher octane product commonly referred to as JP4 or jet fuel.

The "Depends on" comes in with the highly volatile cost of petroleum based fuels.

You can figure that common fuel for piston type aircraft (commonly 100LL / 100 octane Low Lead) is similar in price to automotive gasoline with a $1 - $2 premium usually added by the supplier because you have few choices of where to buy it unless you go to another airport. The cost of going to another airport is usually higher than just paying the premium.

Jet fuel (JP4) is often cheaper than the 100LL for piston aircraft because the airports desire more jet aircraft to do business at their facility.

Currently (Winter 2010) fuel for my piston aircraft is in the neighborhood of $4.50 per gallon and my aircraft (Single engine Piper Warrior) burns approximately 9 gallons per hour.