Overview
The objective is not to minimize cost at any price. It is also not to maximize quality without regard to cost.
A very cheap model may reduce quality too far. A very expensive model may cost several times more while improving the result only slightly.
The desired point is the combination that provides sufficient quality at a sustainable cost.
This can be visualized as a curve with an optimal region. Moving from a weak model to a capable model can produce a large quality gain. Moving from a capable model to the most expensive model may produce a smaller gain while increasing cost substantially.
Large systems may accept a modest reduction in quality if it produces a much larger reduction in operating cost. The correct trade-off depends on the value and risk of the task.