yes, zeolites are cheap, no quesiton. but the zeolites sold as supplements are suspended in water, so i guess you are paying for that process. i mean, if you bought a chunk of zeolite, you would still need to suspend the crystals in water to get the most affect, which i wouldn't have any idea how to do.
but, as you say, i am sure the production costs for these products are extremely cheap. its the triumph of marketing.