How much value does an apple phone lose per year? 20%?
Most people purchase an iPhone for $199 (with a two year contract). While that might seem expensive, the product yields enormous utility if used on a daily basis (as most people do).
Let's say you are the type of user that upgrades your phone every two years. If so, then that iPhone will only cost you 27 cents per day. Considering how much people use their iPhones, not just for making phone calls but for running apps, checking email, playing games, finding restaurants, getting directions, etc., that 27 cents a day is an absolute bargain. Even when you consider your monthly cell and data charges.
Compared to other devices, they actually hold their value well. All electronics depreciate, and generally pretty rapidly, but older (working) Apple phones are in high demand in developing economies.
In my case, I hand down the older model of my phone and my wife's to the kids as glorified iPod Touches when I upgrade. In a couple more upgrade cycles, the eldest will probably get his hand-me-down still activated as a phone.