I think you're looking at this completely the wrong way.
This is like saying you'll only pay your brain surgeon for four hours work because that is as long as the operation you require will take. But that four hour operation is built on a lifetime of training and skill.
The value that you provide the customer is the metric from which you should be deriving your billing. If someone else provides an equally effective solution but takes an order of magnitude more time to deliver it, then you are just that much more efficient.
This is like saying you'll only pay your brain surgeon for four hours work because that is as long as the operation you require will take. But that four hour operation is built on a lifetime of training and skill.
The value that you provide the customer is the metric from which you should be deriving your billing. If someone else provides an equally effective solution but takes an order of magnitude more time to deliver it, then you are just that much more efficient.