I've been thinking about this question. Mr Cat, although I don't have an answer about the cost, I do have a method to determine my course of action.
Assuming the truck makes 500 kmiles
1. Quote the cost of a repair.
2. Quote the cost of a replacement.
3. Determine the value of the truck.
If 1 and 2 exceed 3, then replace truck. 3<1 & 2
If 2 exceeds 1 and is less than 3, then repair. 1<2<3
If 1 exceeds 2 and is less than 3, then replace. 2<1<3
Although I think the chances of the Assumption becoming true are slim.
The final thought.
I do not expect the AT or any transmission to reach 500 kmiles without maintenance.