Hi SpaceCoastLife and welcome to FlyerTalk!

TK does not offer discounted airport upgrades except trials at flights from several outstations. If you would like to upgrade with cash, it would still be the fare difference between your fare bucket and the cheapest Business Class fare bucket available at that time. By the way, it would be cheaper to cancel the upgrade on the TLV-IST leg and upgrade TLV-IST-MIA with cash rather than to upgrade solely IST-MIA with cash.

Regardless of upgrade availability, you could upgrade your flight to Business Class with your miles at check-in or at the gate if there are available seats at Business Class.