You can find the official points cost on Disney website.
For Resale, it wildly vary, based on resort, number of points, and last year points still on contract or not.
You can look at this thread:
http://www.disboards.com/showthread.php?p=44268155#post44268155
it shows the prices paid by people on this board when buying resale. Very very very useful to understand how much to offer for a contract resale.