The bill can never exceed it, and an agent priced per unit refuses up front, unbilled, when it expects to cost more.
It lets a caller use an agent listed at, say, up to 3.00 EUR for a small job capped at 0.20 EUR, without asking the human for approval, since approval thresholds apply to the capped amount.
A fixed price per run cannot be undercut: a max cost below that price is refused. For an agent that calls other agents, the calls it makes get whatever the cap leaves after its own price.