The more residual income you can build, the better off you’ll be. In fact, it’s said that the average millionaire has 7 different streams of income. By creating passive income streams that generate money while you sleep, you’ll build wealth faster and diversify the ways you’re able to make money – which helps protect you from the loss of any one individual income stream.
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.

In equity valuation, residual income represents an economic earnings stream and valuation method for estimating the intrinsic value of a company's common stock. The residual income valuation model values a company as the sum of book value and the present value of expected future residual income. Residual income attempts to measure economic profit, which is the profit remaining after the deduction of opportunity costs for all sources of capital.