Will get a lot of shots, but not as many as he claims. Figure a standard rifle is using between .4 and .5 gr. of gas per shot. IF you never lost any co2 (and you will lose quite a bit every time you stop shooting and bleed that line)...and if that 20 oz. tank was really filled with 20oz (some shops will short fill), would expect between 1000 and 1400 shots. In the real world, are doing good to get 1000shots.
That's still a bargain, assuming a typical $5-$6 refill for that 20oz. tank. My cheapest price on 12gr. would c ost me $19 for that 20oz. (and more common price of 12gr. that would be about $23). Would take an amazing $66 to equal 20oz. of co2 in AirSourse cartridges.
...can figure it any way you like, but 20oz. of co2 is equal to 560 grams. At 12 gr. a powerlet, would take 46 2/3 of them to equal 20oz. At 88gr. an AirSourse can, would take 6 1/3 of them to equal 20oz.