iPhone Retains Value Better Than Android Handsets
/[A]fter 18 months, iPhone retains 53 percent of its retail value, compared to 42 percent for Android and 41 percent for BlackBerry.
Matches up with my experience. I recently (December) sold an iPhone 3G for $115. That's 42 months later and 17.7% of the original, unsubsidized price. 52% of the contract price I paid when I bought it.