The costs were already studied in 2023 and were deemed cost ineffective[0]. The report contained three main strategies (VE1, VE2, VE3) with A & B plans for the first two. Costs would be in the range of 10-15+ billion with 15-20+ years allocated for construction time[1, p. 47].
I agree that a new line at least from Tornio to Oulu would make sense.
There's also a lot of heavy industry in the Gulf of Bothnia, like Raahe and Kokkola.