It's an interesting question, but it depends on what you want to do with the answer.
They have a blended cost which includes
ECB funding at 1.25% (I think, but it may be 2.25%)
Cheap deposits
Expensive deposits to attract new business
Bonds
The question could be asked "Why are they charging mortgage holders 6% when they are paying the ECB only 1.25%?"
But the 1,25% would not be a fair cost. They are under huge pressure to get their loans/deposits ratio down i.e. to increase their deposits and reduce their loans. To attract new deposits they have to pay 3%.
So I would say that their marginal cost of funding is 3%.