Revenue model is part of the business model.
Revenue model, indirect or direct, only covers the way in which the company receives money or compensation from a product or service. What we need to consider is that APIs can play many roles in a company’s business model.
The business model covers all aspects of running a business or a product line, from customer segments, to channels, to operational activities, resources, and costs.
APIs provide a way to improve, change or add to your current business models. APIs are important if your goal is to create an internal and/or external platform. APIs can also help cut costs by re-using and sharing resources or to create a network effect in your business model.
These are all examples of APIs role in the business model from the book API Economy 101 (in Finnish, API-talous 101).
In broadest sense value proposition can be described as a formula where Value = Benefits - Cost (cost includes economic risk).
Developing a value proposition is based on a review and analysis of the benefits, costs, and value that our company can deliver to its customers, prospective customers and other members in the network of our company and within our company.
Treating APIs as digital products means they can be bought, sold and shared. Below is a useful list for assessing the business opportunity with APIs.
Source: Marty Cagan, Silicon Valley and MindTheProduct -network guru of product management: http://svpg.com/assessing-product-opportunities/