They use nickel, not zinc, and nickel costs more than copper. Copper is currently trading around $4/lb, nickel at $13/lb. Then again, the process uses up a relatively tiny amount of nickel relative to the value of the energy produced. The capital costs (producing nickel dust of the required granularity, making an e-cat unit to hold it, high-pressure boilers, turbines, generators, etc) are going to dwarf the price of the raw nickel.
If/Once Rossi’s process is scientifically studied in some depth, I would be “very” surprised if there don’t turn out to be other systems that will do the same thing.