Its kinda hard to gage. My understanding is that Disney will buy a contract if there is a wait list of existing members for points with that use year and they won't loose any money on the deal.
That's why you hear of good deals getting through while others that are higher priced being grabbed by Disney.
What was suggested to us as a guideline is 65+ for HH and VB. 69+ for OKW and 72+ for the rest.