Like anything else your personal circumstances are what makes or breaks the cost, by the time I pay for fuel it costs me about £12-£13 per cyl fill but I probably only fill it once a year.

At that rate even the cheapest on the bay would take 10-15 years to break even, & that's allowing for cylinder test costs,

OTOH for someone who's getting their cyl filled every other month It might pay for itself in 2-3 years, assuming it lasts that long