Picked once for the whole factory, on three facts: it is the cheapest scalable click of the broad platforms at $3.14 against Google's $13.75, it works in a niche that nobody searches for yet because it creates the demand instead of harvesting it, and the creative that worked on product one can be re-tested on product two the same afternoon. Nothing else on the list is all three.
Three customers must pay back one customer's cost, so the price a channel forces is CAC × churn × 3 ÷ margin. Margin is taken as 80%, which is an assumption and the one number here most worth replacing with a real one. A product that cannot be sold at the price its channel forces does not go in the factory.