Based on this description, I see two types of documents that it could be.
As described by motosubatsu, it could be a business case, which describes the feasibility of building a product or executing a project and what the benefits of the successful effort would be.
A vision and scope document, among other things, contains the goals and objectives of the product or project. Savings - expressed in money, time, effort - can be an objective that could be achieved by successfully completing the work.
Both are created early in a product or project life cycle. In addition, taking an evolutionary approach to your documentation could allow for adding additional content from these types of documents, based on stakeholder needs, as the work progresses.