If we are to believe what Priceonomics have to say, then, our phone loses its value pretty quickly, lest we have an iPhone.

According to their research, iPhones have the best resale value, while Android devices' resale value is almost similar to BlackBerrys and is reasonably low. This means that second-hand iPhones are valued more than their equi-priced siblings from Android or BlackBerry stable after a while of their initial purchase.
An 18 month old iPhone still holds 53% of its original value versus a droid which holds 42% and Blackberry, 41%. *A slightly more precise analysis reveals, if you buy an iPhone, you can sell it for $13.20 less for each month you own it. BlackBerrys and Androids on the other hand loose close to 40%.
According to the website, cell phones dont depreciate linearly over time. Most of their value is lost right after you have made a brand new purchase and the rest depreciates gradually. *But at any point of time, iPhone retains more of its original value than Android and BlackBerry phones. Even four years later, you can sell a first generation iPhone for close to $110 dollars.
The site also enlists a few money saving tips and for minimizing your total expense when buying a phone and selling it after a while.