Money usually speaks with a loud voice so I'd present that an accessible web presentation requires less investment to develop and maintain. It's the same point that's often made for the benefits of being compliant to web standards. Something that you'll often see is that one of the benefits of being web standards compliant is increased accessibility.
For any presentation, simplier is better. I recommend always starting with the simpliest and then adding complexity only where it's needed because each layer of complexity requires more elements, increased costs, to keep it accessible. Text > Images > PDFs > Audio > Flash/Video. The simpler your presentation, the less it costs to develop and maintain, while making it easier to increase your user's ability to access the content.
Christie Mason