" ... which already cost business more than $100 billion a year."
Firstly "which already are a cost" should be "which are already a cost", but I don't think that's the problem.
Perhaps you are right.
In any case, if B is correct, they are stating that
"The effects are not a cost to business." But, "The effects do cost business."
I'm not sure whether that's a difference. Both are grammatical to me.
Student or Learner