If you thought Apple (NASDAQ:AAPL) was cheap last week, it's even cheaper this week, thanks to the Street's disappointment in the company's underwhelming first-quarter results. After Apple's quarterly figures hit the newswire, Apple lost a whopping $50 billion of its market capitalization in a day's time. Is the sell-off a buying opportunity?

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The results
At first glance, Apple's results appear to be about in line with consensus analyst estimates. Analysts had estimated Apple would report $14.36 in earnings per share on $58 billion in revenue. Instead, Apple reported revenue of $57.6 billion, and EPS of $14.50 -- a slight miss on revenue, and a slight beat on EPS. Not bad.

So, why did shares fall about 10% in the coming days? iPhone sales -- Apple fell 4 million units short of estimates. Analysts had expected about 55 million units sold, and Apple sold 51 million.

Why is the street disappointed with 51 million iPhone sales?

Growing, or not?
Apple's a big company. Actually, that's probably an understatement. Apple is an enormous company -- the most valuable publicly traded company in the world, in fact. And it's this impressive feat that has investors concerned.

After more than a decade of domination in at least one category at a time, including MP3 players, smartphones, and tablets, investors are wondering how long Apple's streak of blockbuster products can continue. Can Apple continue to grow? Can it even maintain its current level of profits? As the famous saying goes, "What goes up must come down."

Even more, the majority of Apple's monstrous profits come from one product category -- iPhones. Not only does the category account for more than 55% of Apple's revenue, but it's also the most profitable product segment. This means that any apparent slowdown in the segment's growth could signal a red flag.

In Apple's first quarter, iPhone unit sales grew 7%. In the year-ago quarter Apple reported 29% growth in the segment. The significant slowdown spooked investors. Moreover, Apple's year-over-year comparisons in Q1 benefited from an iPhone rollout in China, along with the U.S. rollout in 2013, unlike in 2012, when the iPhone didn't come to China until the end of December.

During Apple's first-quarter earnings call, analysts pressed Apple with questions concerning the management's expectations for growth.

Barclays analyst Ben Reitzes asked Apple sternly, "[...] are you still a growth company?" While Apple CEO Tim Cook responded by saying that they are "very confident of growth year over year." The answer wasn't enough to soothe the Street's worries.

Reitzes' concern has merit. For the full year of 2012, Apple's smartphone sales increased 12.9% compared to the industry's growth of 38.4%, according to IDC. 

Susatainability
But maybe the Street is asking the wrong questions. While growth would certainly be nice, Apple doesn't have to grow to reward investors at today's conservative valuation. It simply has to build a sustainable system to maintain current profit levels.

On that front, Apple seems to be making improvements. Thanks to a narrowing gap in Apple's year-over-year gross profit margin, and a generous share repurchase program, Apple was able to grow its EPS in Q1 after reporting year-over-year declines for four quarters in a row. 

Even more, loyalty to Apple's products remains as high as ever. Despite an onslaught of lower-cost Android devices, competition hasn't weighed on iPhone retention and customer satisfaction. For instance, a recent study by Kantar suggested that the iPhone loyalty rate is about 90%. And a December survey by ChangeWave indicated a 96% customer satisfaction rate for the iPhone.

Value matters
With a $158.8 billioncash war chest, an aggressive share repurchase program, a loyal customer base, and EPS on the rise again, Apple stock offers meaningful value for investors, given its conservative valuation of just 12.5 times earnings. Sure, iPhone sales are slowing. But value puts everything in perspective. With more than $50 billion in market capitalization erased from Apple's value, the sell-off offers a solid entry point for investors with a long-term investment horizon.

Daniel Sparks owns shares of Apple. 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.