Ideally, it would be a completely automated system. You don't need to understand machine learning to know that it would be better for the computer to do everything itself without human intervention
Simplified: Machine learning algorithm constructs page to show human.
Humans click on this or that on page.
Click data is fed into algorithm.
Algorithm uses this data decide how to show better page to humans
You are taking human input. It is still completely automated, it requires no human intervention.
Also, humans are prone to error and many other inefficiencies. They get sick, quit, fluctuate in performance, require attention and care... you say "cost" like it's some easily calculable number, but it's a whole boatload of intangibles that just... disappear if the computer does it.