The costs that get payed per prevented migraine in option B are irrelevant. The value of a prevented migraine isn't determined by the price that you pay to prevent a migraine.

The difference between option A and option B is that A prevents 20 additional migraines a cost of 250$. This means $12.50 per migraine. What kind of migraine are we talking about? A duration of 3 hours and involve intense pain, nausea, dizziness, and hyper-sensitivity to bright lights and loud noises.

$4.16 per hour of suffering migraine is lower than minimum wage. Normal minimum wage happens at a time that you can shedule in advance. You can't shedule your migraines in advance. They are likely to happen during the times where you have the most stress.

Being occupied with the migraine however isn't the only thing. Intensive pain also matters. You don't want people suffering intensive pain without good reason. Letting someone else suffer intensive pain is morally torture if you are a utilitarian.