U.S. stocks are higher on Thursday, with the Dow Jones Industrial Average (DJINDICES:^DJI) and the broader S&P 500 (SNPINDEX:^GSPC) up 0.21% and 0.27%, respectively, at 12:45 p.m. EDT. The Nasdaq Composite was up 0.37%. 

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Professor Aswath Damodaran of New York University's Stern School of Business has written the book on valuing common stocks -- four of them, in fact. He is arguably the world's leading authority on the topic, and he's been valuing Apple's stock on a quarterly basis for the past four years. As such, when he posts an updated valuation on his blog, I read it immediately. Furthermore, academic expertise is not his only credential regarding Apple. 

Pr. Damodaran invests has been buying and selling Apple shares based on the results of his analysis and has made some prescient calls on the stock in his blog. For example, on Jan. 27, 2013, with the stock at a split-adjusted price of $63.26, he wrote, "there is a 90% chance the stock is undervalued," and put its value nearly 40% higher, at around $87 (split-adjusted, or $609 per share pre-split). Over the next 24 months, the price of Apple shares would double. 

This is his current thinking on the stock's valuation: 

The last earnings report from Apple, which came out in late July, contains few surprises or twists, with perhaps the only surprising feature being the unexpectedly large jump in the cash balance to over $200 billion. That fact, while Apple continues to buy back billions in stock and pay large dividends, is a testimonial to the cash machine that Apple has created with its iPhones and iPads. In fact, just incorporating the higher cash balance and the lower share count into the valuation yields a value per share of close to $130. 

At $130, his intrinsic value estimate is in line with the stock's 52-week high ($134.54), which it achieved at the end of April. The stock's 14% decline from that level provides investors with a margin of safety on the shares of a company that is once-in-a-generation: 

At the time of this post, the mood around Apple has darkened and the stock has dropped to less than $110. The purported reason for the stock price drop is the slowdown in Apple sales in China, but that sounds to me like an attempt to fit a "good" reason to an old-fashioned sell off. Even allowing for a Chinese economic slowdown, Apple is starting to look like a bargain to me again, but given the ebbs and flows in momentum in this stock, I would not be surprised if this round of selling leads the stock even lower, before good sense prevails. I think I will wait a few weeks before putting my buy order in, but it looks like I will once more be an Apple stockholder. 

It's not every day you're handed the opporunity to buy into one of the world's greatest businesses at a discount. Patient, value-driven investors ought to seriously consider opening a position (or adding to their existing one) at current prices. I expect those who follow through will be able to look back happily at their decision in three to five years' time (and beyond) -- and their returns will hardly be academic.

Alex Dumortier, CFA, has no position in any stocks mentioned. The Motley Fool recommends Apple. The Motley Fool owns shares of Apple. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.