My suspicion is that TK sometimes targets you specifically for cash upgrades even if no tickets in business are available for purchase in the booking tools if it predicts that you will likely be upgraded on that flight. Like tricking you into paying what you would get anyways with operational upgrade. I know it seems like a conspiracy theory, but I see weird things in relation to my booking. Let me get back in a few days. But in the past I noticed that TK also offers Business upgrades for cash when the flight is not that profitable (not a lot of Business sold and Economy just above 1/2 full). But it is my hunch for now.