Much of the 'cost' to run will come down to how much power you are drawing from the generator.
I found this on the model that you posted.
The fuel tank is 6.3 Gallons (24 litres), so compare that with how much you get in the cylinders.
At the full rated draw of 4500W it says it can run for 7.5 hours, whereas at half draw (2250W) it can run for up to 11.
I only have basic generator knowledge but hope this helps :)
Thank you I appreciate the knowledge