This commentary originally appeared Dec. 22 on RealMoney -- the popular investors' source for comprehensive financial coverage and insightful analysis. Click here to learn more.NEW YORK ( RealMoney) -- Apple ( AAPL) may be the most widely held security among the investing public today, and for good reason. Year to date, shares are up nearly 25%, while the Nasdaq is roughly even. Google ( GOOG), another widely held name, is up less than 10% year to date. Since 2008, Apple shares are up over 100%, while the Nasdaq is up around 5%. The further back you go in time, the greater the return Apple has delivered for its investors. Shares in Apple have single-handedly saved many a portfolio. Yet despite the phenomenal past performance, the stock is likely not done delivering for shareholders. However, from here, Apple will not continue to generate that same rate of return. And no stock goes up forever. At some point, the growth curve flattens, and the stock price performance follows. Since investing is all about opportunity cost -- placing investment capital in areas that are likely to generate the highest rate of return with the lowest probability of loss of capital -- an real investment in Apple occurs only when it can be bought significantly below intrinsic value. While headlines tout that Apple shares are worth $600, $800 or $1,000, investors can arrive at their own figures by performing a discounted cash flow model. In fiscal 2006, ended Sept. 30, 2006, Apple earned $2 billion. In fiscal 2011, Apple's net income was around $26 billion, an annualized growth rate of nearly 70%. Over the next five years, Apple's growth rate will not come close to 70%. But even off of $26 billion, I believe Apple can grow its net income by 20% over the next five years, but even that rate can easily be hurt by any product mishaps. I will discount those earnings by 10% and assume Apple's residual business value is worth 15x 2016 free cash flow discounted back to the present. >>Learn about this premium service that hands investors investment strategies from veteran Wall Street pros.