How much cheaper? You can hardly get humans to work two shifts a day, let alone three.

Only sometimes and costs change over time. The first robot is almost always more expensive than a human, but the second robot comes after the design is done and so it generally cheaper than the human (accountants will figure out how to amortize these costs and thus give us a better picture of costs.)

Robots also get cheaper over time because we learn. You can buy many parts in bulk including computer libraries to control them. You can find many people who know best practices who will not make some of the early mistakes that cost money.

Only if you don't need to pay them a living wage.

In China, yes, but not here.