Herman Holtz and Peter Meyer, writing for The Independent way back in 1996:
Start with what you are providing. Are providing a task (such as programming, DB design, project management, specification writing and so on)? Or are you providing an effect (such as more transactions per hour, lower cost per sale or invoice for your client, faster time to market)?
If you are selling a task, you will tend to charge less than if you are selling the effect of that task. This is probably fair. We all expect to pay more for results. Perhaps more importantly, clients often know what those results are worth before you start.
Selling a task is often easier than an effect, but that does not mean that it is more fair.