Remember that this is a collaborative assignment. Obviously, everyone in the group will have to compromise about the topic. Although you are free to write an external proposal, an internal proposal is less complicated, for it does not call for you to demonstrate that your organization is reputable. You are seeking permission to carry out a research project in exchange for the organization's granting you time and the other resources. The key to writing a successful proposal is to show an understanding of the readers' needs.
Audience analysis is critical here.
Most often, the way to demonstrate a problem or opportunity is through cost figures. In an appendix, provide the complete calculations for your claim. Integrate your research into the discussion of the problem and the rest of the proposal. Perhaps the most common deficiency in student proposals is the failure to carry out secondary research and to use it in framing the discussion of the problem or the opportunity. Don't just toss in a few citations to the literature.