NGV is only cost-efficient if you're a heavy user. I remember doing this calculation some time back, and unless you hit a certain mileage per year, you won't see much of a difference.

That's assuming the wear and tear on the engine remains the same as using petrol, which isn't true. NGV problems include more frequent gasket replacements, as well as problems relating to the air mixture etc. It's not so cut and dry. I had the same initial thought as you until my dad pointed out all these things that I overlooked.