Many of my prospective clients think that the cost of presentation design is variable: the longer the presentation, the more slides, the more expensive it gets. 

The cost of presentation design is actually pretty  much fixed. The big cost is getting to know the company, the story, the science, the technology, the market. Once you have done that, designing the actual slides is not that much work anymore for a professional designer.

So the most costly presentations are usually those where the story is not very clear.

Art: Marinus van Reymerswale, The money changer and his wife, 1539