Microsoft’s idea of a deep discount for this game is 50% off. I would bet that FM23 will be delisted before it sees a bigger discount than that.
I was originally in the same boat as you: I was a Game Pass (Ultimate) subscriber, and bought FM’s Ultimate Edition upgrade. I realized I wasn’t getting the value out of GP that I once was, and realized I had a bunch of rewards points saved up. I used those to buy the base game for essentially nothing when it went on sale for $35. I then canceled Game Pass altogether. Maybe you could do something similar.