Costs depend on the generator’s size, fuel type, and local gasoline, propane, diesel, or natural gas prices. Nevertheless, the average cost of gas is $2.5 dollars per gallon. Everything You Need To Know About Installing A Backup Generator Standby Generators Generator Sales Service Installation Is $3.25 per thousand cubic feet. How much does it cost to run a generac generator on natural gas . Natural gas, liquid propane or diesel also regular gas.natural gas if your generator runs on natural gas, it's hooked to a supply provided by your utility company. A generator that runs on natural gas will cost you slightly more, averaging around $2,000 to $5,000. Please note that this table is intended to be used as an estimate of how much fuel a generator uses during operation and is not an exact representation due to various factors that can increase or decrease the amount of. Generators can run on liquid propane, natural gas, diesel, or gasoline. Cost of diesel generator o...