It is 8*# of Steps in Path.

So a Son of Niefel starts with 2, and you take them up to 9. The costs are:

2 - 3: 8
3 - 4: 16
4 - 5: 24
5 - 6: 32
6 - 7: 40
7 - 8: 48
8 - 9: 56

Total: 224 DPoints

For a Frost Father they have to go one more step up, since they start with 1 Water.

Add another 64 DPoints to the total to get him to Water 9.

This is why it is usually only feasible to get a single "9" Blessing and usually only with units that at the very least have 2 in the initial Path, and best case, 3.