I think it's a pretty questionable assumption that the utility difference between 0 and 1 migraines a year is significantly greater than that between 10 and 11. Both are infrequent enough not to be a major disruptor of work, and also infrequent enough that the subject is used to the great majority of their time being non-migraine time.

Headaches avoided per unit money isn't a very good metric; by that measure, a hypothetical medicine D which prevents one headache per year, and costs a dollar, would be superior to medicines A-C. But medicine D leaves the patient nearly as badly off as they were to start with. A patient satisfied with medicine D would probably be satisfied with no medicine at all.

The metric I used to judge between A and B was to question whether, once the patient has already paid $100 to reduce their number of headaches from 100 to 50, they would still be willing to buy a further reduction of 60 hours of headaches at a rate of about 4.16 dollars per headache-hour. My answer was indeterminate, depending on assumptions about income, but I chose "yes" because I would have to assume very strict money constraints before the difference between A and B stops looking like a good deal.