I asked about this in connection with changing the loan term/installment, and OP is offering to lower the margin “slightly” (I still need to ask exactly how much..), and the fee for this would be €200 + changing the loan term from 25y → 35y, which costs €150, so €350 in total for the changes.
I guess that’s a pretty standard price level for these kinds of changes.
EDIT: Margin 0.43 → 0.35%.