IMO, every point missing from the 2016 and 2017 use years is worth $10 - $12. So I'd only offer the list price less $12 * Number of missing points. If the seller is offering to reimburse dues, I'd back it down to asking less $6 - $7 * Number of missing points. I've assumed list price is about what the contracts seem to be actually going for based on recent ROFR data.
For example, if it's a 100 point contract and the seller wants $11,500, I'd offer $9100 (11,500-(200 missing points * $12/point)).
Remember the owner of record in 2017 (you) will be responsible for the dues for those missing points. Some sellers will offer to reimburse for future dues if the points are not available - that means the future dues are estimated and the total price of the contract is adjusted accordingly. The estimates may turn out to be good, but they may be off quite a bit. One never knows. Seller should have already paid 2016 dues or pay the remaining months if paying monthly. I would not pay dues for points I am not getting unless the price was adjusted accordingly.
In the end, it will come down to what the seller is willing to accept. For some of the smaller contracts at the near park resorts, the sellers have been getting more than what I personally think the contract is worth. Some owners will negotiate and some will not.
So bottom line - offer what you are willing to pay. Be willing to walk away if the seller wants more than you think the contract is worth.
Good luck!