Michael,
Be careful in these discussions to draw the distinction between Yield and ROI. Everything that was said above about using a financial calculator such as 10bii to calculate yield is true, but this yield is not the same thing as ROI as most people think of it.
When people talk ROI they are often thinking of compound annual growth rate (CAGR), such as the stock market, or a bank savings account. The big difference here is that interest on an amortizing loan such as a mortgage is charged on a smaller and smaller amount as the principle balance is paid down. Whereas in your bank savings account you are paid interest and the balance stays the same and actually goes up as you earn interest on your interest, hence the word "compound".
The problem with confusing these is if you promise an investor an 8-12% return, they are going to assume you mean compound annual growth rate of their money (wouldn't you think that?). But if you buy a discounted note where your yield is 15% and you promise the investor 8%, you will actually be losing money. Unless you are very clear in that you are offering them 8% yield, which is not the same as their original investment growing at 8% per year.