I've got a generator, and I'm wondering since my monthly usage is 719kWh how much would it cost to run the house per day if fuel is $4/gallon?
The generator you linked to claims to need 4 gallons of fuel to produce 9.5 hours of power at 2.5 kW, or 23.75 kWh. That is about 6 kWh per gallon. At $ 4 per gallon, your fuel cost is about 67 cents per kWh.
In other words, you would spend about $ 16 per day on fuel to average 1 kW. That cost does not include lubricants, maintenance, depreciation, nor the hassle of refueling the generator on a daily basis. In most places, this fuel cost is much more than the retail cost of the electricity produced.