The reward for me was completed with a one-time-payment. There won’t be any newly Duke household formed after crushing the Duke Hertha household either. Watching it on long-term, it is helping the national budget.
The annuity of a duke is 20 platinum coins per year.
Considering that it finished with never having to pay those yearly 20 platinum coins to Duke Hertha any longer, it might have been a cheap deal to reward me those 50 platinum coins.
Additionally, as there were ten-odd vassal households connected to the Duke Hertha household, those had to pay a large penalty for the sake of avoiding being amongst those getting dissolved.