Generating your own leccie is 4 x the cost of a line supply.
Can be, not is, I would say.
My 4.5kVA diesel genny goes through about a litre and a half of (red) diesel while running a 3Kw electric fire. So about twice the cost of mains lecky. Small petrol ones will certainly be more costly than one running on red diesel, but the modern inverter gens only rev to what power is required, so not running at 3000rpm continously. Saves on noise, fuel and engine wear.
An old Lister CS could be far more economical on fuel – especially if fed on chip shop waste oil. But not an option in this scenario (perhaps unfortunately).