Your point is fair, but they know order of magnitude. They know you're (likely) not bringing $10B annually of value, and they know it's more than your current billing rate (or they wouldn't waste time with you). The point is to talk with the client and try to pin the number down to bring your rate closer to the value.

Somebody in the enterprise is responsible for ensuring that projects aren't run without any expectation of creating value; that's the person/group to talk to. If you're not talking to that person/group, then you're right that value-based pricing is going to be hard/impossible to pull off.

If the buyer doesn't have any idea how much value you can/are creating, then your first task is to help them quantify the value (this is good for them no matter who they hire).