Bit of 'how long is a piece of rope' question.
Depends on the quality of the phone and how easy it is for people to get hold of one. And how long I keep it depends on when the next whizzy toy appears and how long I can resist.
Generally though, I'll get back between 75-90% (on the first iPhone actually made a profit before it was available in the UK). You do, however, have to deal with the ever-growing number of hassles on selling on phones in places like ebay now. It's all a bit of luck and timing really.
So, after you've purchased the first phone, it becomes a much smaller payout to get the new best thing once you've sold the previous one.