Resale is simple.... there are some ppl who bought it for 95 thousand and carried the 3GS exclusively for three months or so... it then gradually declined to 75 and then the 3GS has been at the 50-60k range for the longest i have seen for any phone. Pin packed pieces are still not below 52-55 for 16GB.
Currently in khi... one can buy a 3GS 16gb second hand in the 40 to 46 thousand bracket... depending on condition it cud be slightly lower too.
A new iphone model will be out by may or june. By the end of the year a good second hand 3GS will still be around the 30 thousand mark + - a lil bit.