Not only does the salt water need to be changed regularly, but the metal electrodes also need changing every so often.

2 less costly metals could make this more cost effective. Maybe aluminum from empty beer and soda cans for one electrode and empty food cans (that are not aluminum) for the other electrode could provide very inexpensive electricity.

There are also many possibilities for the electrolyte solution, like soapy water from showering, or urine, or water with lawn clippings or etc.

While these may be less efficient than the copper-zinc-salt battery, they could be more cost effective.