Originally Posted by
sbams
If you look at the miles price for a one-way in Business on your travel dates that may give you an indication. Of course others will have recent data points too.
CASH price, not miles!
Miles have already been used to make the booking; miles cannot now be used to upgrade.
To the OP: upgrading an economy miles ticket is not a good way to get into the J cabin. You're basically last in line to get the chance to upgrade, and if you were to upgrade if there even was availability, you'll have ended up paying more, in total, than most other ways of travelling in J.