The gold plating on PCBs is very thin. It's usually well under a micron thick. Even in 'hard gold' plated PCBs, the thickness may be under 2 microns. You would need something like a couple of dozen of these devices to yield a full ounce of gold. That gold would still need to be refined to get it to a high enough purity though as it is often mixed with cobalt and nickel. It's worth it if you have a lot of gold plated PCBs to recover with, but it's not going to make you rich.