Mark is correct. But it takes a different mindset in approaching the customer. If you let them keep the idea, ""this is a product enhancement,"" then you're stuck. If you insist that they have a business process-functionality issue, then you charge to fix that problem -- which might or might not include software. Kless talks about the ""FORD consulting model"" (http://bit.ly/1jK2oFP), then you can charge for a modest project to define the problem and produce options (F and O). work the demo into that project if need be.
I have done this more than a few times, and if you take the effort to get the customer to think about the challenge from 20k foot level, you get everybody into a better place.
But it is easier than getting them to agree to, ""The fee for the demo is $xxx"")