Designers tend to bill by the hour, which is why they describe a generally drawn out process by which they arrive at their conclusions. This stems from the fact that they feel they have to "show their work" in order to generate value. Yet as many designers can attest, the big idea is often generated within 30 minutes of hearing the problem. Their accumulated experience allows them to understand what they need to do in an instant. But you don't want to call the client 30 minutes after a meeting and say, that will be one million dollars please. I mean you didn't do anything! It didn't take any time! Except that it did. And in the end your time isn't what they're paying for, the time they're paying for is their own.
First off there's the Paula Scher quote "it took 30 minutes and 30 years. Designers who have dedicated time to the profession can work quickly, but that by no means infers that the idea didn't take time to percolate. More important is the fact that the client is really paying for their time.
They are investing in their brand and their voice. When done right this can be something that holds incredible value for a long time. Hence they aren't paying you for your 30 minutes, they're paying for the years of value their brand gives them. At this point what would a rival big box retailer pay to achieve the brand value of Target? This value exists on a smaller scale as well.
Designers don't need to be afraid to charge good money for their service. Clients are getting something significant in return. Are there sometimes more important things for some businesses to spend their money on? Yes. Do designers feel as though they are in competition with those things? Yes they do. This is why designers need to stand strong in the storm of questions around their value and work with clients to help them gauge how important their brand truly is. If it makes a a big enough difference, a client won't care if the idea came to you after weeks of research or like a lightning bolt while you were on the john. Time is a valuable commodity and the more longevity their brand has the better the deal looks to them all the time.