$0. It's taxpayers euros.
But it's not inefficient if this fixes a chicken-egg problem: overcrowded big city and underpopulated rural area sucks.
Nobody can/wants to go in rural area if they can't make a living there. A fiber speed Internet unlocks lifestyle for many.
It's the same as investing in say a high speed railway between major cities A and B and adding a middle-of-nowhere stop C midway. That C then can flourish to become a medium/major city... So while the initial investment looked like a waste, it's beneficial to provide a better life for people living in both A,B, and C.