It's not the cost of the switch. It's the cost of assembly (wiring). And the switch must fit the form factor.

The cost lies in replacing the 300$ phones where the switch breaks (earlier than expected).

Cameras are easier, blocking light is even more low-tech than closing a circuit. Unfortunately, cameras are already the limiting component in the current craze for ever thinner devices, so adding a slider on top would not be very popular. Theoretically, cameras could also be physically disabled by shifting some elements that are already there out of the optical axis. The precision requirements to reliably restore it to working condition would be insane.

And if the switch is too prominent people will whine about accidentally switching things off when they didn't mean to. So the switch would get moved out of the way, and pretty soon people would just turn it off (if it's even on by default, which it probably won't be) and leave it off, because most people can't imagine why anyone would bother to exploit such capabilities.