My e-product is currently in development, but I think it could reasonably bring in 1/25 of my income in the first year with minimal promotional effort. If it takes off, maybe 1/5 of my income. Building that up so that I could direct all the proceeds to paying down student loans is a great incentive. I need to focus there, but I also have a few other non-traditional digital products in mind. I need to test the market there before expending too much time or energy.
Writing and publishing an eBook has become a widely popular means of earning passive income in recent years. eBooks on non-fiction topics like online marketing, business development, career advice, and the like are especially successful. While fiction eBooks are also good sellers, the market for them is much more competitive as compared to non-fiction ones. Once you've written an eBook, you can put it up for sale on platforms like Amazon's Kindle Direct Publishing or Apple's iTunes Connect.

The rationale for the residual income approach is that it recognizes the cost of equity capital in the measurement of income. Accounting net income includes the cost of debt (i.e. Interest expense), but does not reflect dividends or other equity capital related funding costs. This means that accounting income may overstate returns from the perspective of equity investors, conversely, residual income explicitly deducts all capital costs.
Residual income is the amount of net income generated in excess of the minimum rate of return. Residual income concepts have been used in a number of contexts, including as a measurement of internal corporate performance whereby a company's management team evaluates the return generated relative to the company's minimum required return. Alternatively, in personal finance, residual income is the level of income that an individual has after the deduction of all personal debts and expenses have been paid.