I know what your meaning on the costs of silver wire - as I too have contemplated the same - though I think the stainless cathodes reference stemmed from another setup in-where the anode was consisted of 6 x 8gauge silver rods, whereas the cathodes were stainless to help cope with wire-size shape and sizing as well as costs. And so, I'm leaning toward a typo insofar as the BOM goes on this particular setup.
As for pricing, I see the very same pattern in our own health food sections. ie, the last time I looked, I found clear Colloidal Silver labeled as 50ppm going for 65 dollars CAD. Which imo. is nothing short of fraud. And only goes to show the overall state of the health-food industry, not to mention, the most likely reason why it is slowly being overtaken by regulators :(