If we rate Cucu as good enough to be our 1st choice LB in the big games for years to come, we'll pay 50m, why not? But we may have doubts that he can make the LB position his own. Why pay 50m for him, if he is seen as a very good option to rotate Cancelo and Walker?
If we are reluctant to go above 45m, we don't see Cucu as a guaranteed starter but just as an improvement on Zinchenko. Maybe we do not rate his potential as world-class and think that at 50m we can get a player with a higher ceiling.
We lack enough info, but we may be certain that Txiki does what he thinks is the best for the team long term, and not just next season. Had we signed Kane for 150m, we wouldn't have Haaland now...