My experience is that the Airmiles 7 day and 5 day flex tickets are not the same as the ones Disney currently offers (Airmiles confirmed this before my trip, YMMV). I used an Airmiles 7 Day flex at Disney in May as soon as they came available and it was valued at $469 USD after tax. Your experience may differ, I have been waiting for others to post their experiences at Disney guest services but so far have not seen any other posts. So I would imagine, an Airmiles 5 day flex ticket will give you around a $450 USD after tax credit to an upgraded ticket on Dec 12. Once others post, this can be confirmed or adjusted.
Are you Onyx? I did a Personal Shopper quote, 8900am for a current 7 day flex ticket, you could request a quote for a 7 Day non flex which should be less. I suspect when Airmiles has the next batch of Disney tickets available, the 7 day flex will be closer to 8900am than the current out of stock 7 day flex that was 6400am, unless by some very small chance Disney comes out with Canadian tickets in the fall. And unfortunately this is all "wait and see" as to what Airmiles will be able to offer next.
You have time on your side, Dec 12 is another 5 months.