Hello, I know it would be much expensive to run on generator rather than grid power. Now to answer your question how much it would cost?, it would depend on how big the generator is.
Now, the size of the generator would depend on your power consumption plus an extra power to be able to start heavy loads such as air conditioning or refrigerators.
Normally, a small 3 kva generator using petrol would use about a liter an hour. So if you can compute how much a liter of gas cost in your area, multiply that with 24 hours to get a days cost using a generator with a capacity of 3 kva.
A 3 kva can run about 500 watts of lightings, a couple of electric fans, a small microwave, a small refrigerator, NOT a water heater, NOT an electric clothes drier, but could run a gas ( lpg ) clothes drier. NOT an air conditioning even the smallest 1HP.
Regarding Noise, I have seen 10 kva- 20 kva inside a sound proof shell which you could barely hear while running. But when they get bigger than 20 KVA, then it would be harder to minimize the noise as to not to disturb the neighbors.
Then smaller ones like the 3 -7 kva, if they are not inside a sound proof shell, then they are noisy.
Let me know if you still have questions.