Odds are that you will not be able to get a discount at BCV. Disney only retains 3% of the rooms to use for their own use, with the rest being given to
DVC members. Your best bet would be to try 60 days or less from your last day. That's when any unreserved rooms by DVC members is released for cash reservations. That doesn't mean a discount will be available, just that would be your best bet to find availability. Also, after Thanksgiving through the 2nd week of December is the most popular time for DVC resorts. It is the "cheapest" time for members to go and with all the holiday decorations, the DVC resorts are always full with members. It is difficult for members to get reservations without MUCH advance notice, so if you can get a room, and this is where you want to stay, book it at rack rate. Your other option would be to go to the ren/trade board here on the DIS and see if you can rent points. That usually works out to be cheaper than rack rates, and usually in-line with AP rates. It would guarantee you what you're looking for.