Demand for Apple's iPhone 6 remains robust.  Image Source: Flickr user Kārlis Dambrans

When it comes to prices, it's safe to say Apple's (NASDAQ:AAPL) host of iDevices aren't cheap. From a pure cost perspective, Cupertino's been able to charge more through its build quality and brand cachet. The ability for the company to grow device sales while maintaining its premium-pricing strategy is a key reason for the company's tremendous stock performance.

Of course, price alone doesn't account for value. Cupertino's been able to design an amazing ecosystem that allows the company to charge more for its access devices -- iPhone, iPad, and iPod -- while still providing perceived value for consumers.

But judging Apple's value, especially the iPhone, on the basis of retail cost is somewhat misleading. Unlike most products, many iPhone buyers receive a generous device subsidy (roughly $450) to induce them in two-year wireless contracts, significantly lowering the device's out-of-pocket cost. And even if users pay full cost for the device, Apple products have a strong secondary/resale market. A higher resell value lowers the total cost of ownership, and early in product cycles it has the ability to help gauge demand. And in further good news for Apple investors, it appears the iPhone 6's resell value in the secondary market is higher than its predecessor.

Piper Jaffray calculates higher resale value
According to Piper Jaffray Apple analyst Gene Munster, the current-generation iPhone 6 is performing better the previous iPhone models on both eBay in the U.S. and Taobao in China. According to his data, used iPhone 6 and iPhone 6 Plus models are selling for 72% (eBay) and 73% (Taobao) of full retail price nine months into this iPhone's launch, whereas the iPhone 5S and iPhone 5C only sold for 60% and 48% on eBay at the same period in their life cycle. On Taobao, the iPhone 5S and iPhone 5C sold for 65% and 53%, respectively.

While Munster admits it's "not a perfect science," it is a peek behind the curtain and indicates continued strong demand for the product. Theoretically, in the secondary markets, supply and demand should determine the price among buyers and sellers -- thus, higher prices in relation to retail prices point to strong demand. China's results are somewhat noteworthy, as traditional methods of gauging demand in the large, developing country have been lackluster at best.

China is the key to growth
When it comes to Apple's growth, more recently it's been a China story. For example, in the last two fiscal quarters, Apple's reported first and second quarters, revenue in the Middle Kingdom has grown 70% and 71%, respectively, on a year-over-year basis. Matter of fact, when it comes to total growth, China is responsible for 39% of Apple's total revenue growth on a year-over-year basis in its fiscal first quarter and a massive 56% of its total growth last quarter. For a graphical representation of Apple's revenue growth in China, see the graph below:

Source: Apple's 10Qs

As far as forward-looking estimates for iPhone sales go, Munster estimates the company will sell 46 million iPhone units in the current fiscal quarter. That's a little lower than what other analysts are predicting, with some analysts expecting 50 million units or more, but would still be a large year-over-year increase of units, as Apple sold 35 million units in last year's corresponding quarter. At the very least, Munster's data indicate demand in China remains robust, and that can't be a negative for long-term investors.