My idea is to call businesses listing in the Yellow pages etc and propose that they let me build them a home page as a sub-directory on my site that outranks them for the main keyword.

With SEO and great content, it should be possible to also rank above the Yellow pages style of directory that doesn't even provide an email contact.

So, how much could be charged for this kind of service? $997 + x per month

Another idea would be to approach the business with an idea (obtain permission, photos etc.), build the sub-directory, get it ranking well, and then get back to them to see if they want to pay to keep it up.