I offered to write up the backstory for one of the faction leaders, along with a description of that particular faction's neighbourhood. In my opinion, this is a good way to show a new customer that you care about their project and their budget, and that you're not just trying to grab their money upfront. At the same time, this small job is a paid service, as it's only right that my time spent on the project should be remunerated. Even if we decide to part ways after the proof of concept, the customer is left with original content that they own the copyright to and can use for their project.