The English sounds a bit off, you need a comma after expensive too.
"It is expensive, but a necessary investment" just sounds better to me.
I reckon "it is an expensive but necessary investment" is a more accurate translation, but then you'd want to nix all the commas.
Shouldn`t there be a comma after nödvändig, to close that phrase off?
No, that will not be needed. Actually the comma there could be left out too.