The media is citing 1.3 million in Illinois alone with diabetes requiring insulin (out of about 12M population; over 10%) and at least some cases where the cost is over $1000 per month. Assuming it averages out to $300 per month as you said, however, that's still $20/mo. extra per person assuming the difference is distributed uniformly across all insured individuals, and that all individuals in the state are insured.