Hi Laura,
I think for this question I would evaluate what has been given within the context of this problem.
1) "it is worth $500 to prevent one headache day" -- this tells us the value of preventing one headache day, right?
2) I think you are exactly right to focus on the units to ensure you are comparing "apples" to "apples". And you will have that value already calculated by part d.
I hope this helps!
Sam