How much does your propane forced air furnace cost per hour to run?

That would depend on its size or input rating. You can get the input rating off the unit nameplate and divide that # by 2500 and that will tell you how many cubic feet of propane vapor the furnace burns per hour. If you buy propane by the gallon ask your supplier how many cubic feet of vapor a gallon of liquid propane yields and you can figure your answer.