I'd say at least 50% after one year. I've only had stainless steel models and have sold 2 of them so far when upgrading.
For both the original and Series 2 I purchased them at launch for 779 + sales tax (13%) - $880 total.
Funnily enough, sold both of them for the same price when the newer model was launching for $550. So got back 62.5%.
This year I think the SS Series 2 will hold some resale value as the only SS option is the LTE model which is priced a bit higher, and the Series 2 still performs well.