OK, there are a few bits of information you are missing here:
1. How much $ did Vertex put in to developing this drug over the past 10 years? They haven't made a single dime on the drug for a decade, but have been paying some of the top scientists to conduct research; creating manufacturing capacity; and funding 3+ clinical trials to assess the safety and efficacy of the drug
2. How much does it cost for you to be in the hospital each time? If you're in for 2 weeks, it could be anywhere from $100k-$200k each stay. Most CFer's are in the hospital 2x/year at least
3. Your calculation is incorrect - that's not $492 million profit. Kalydeco is FDA approved for 4% of CF patients. That's only 1200 US patients (drug isn't approved anywhere yet). 1200 patients * $300k is 360,000,000 REVENUE. Not profit. You have no idea what their costs are - plus the high cost of all the work they have done over a decade to discover this drug
4. They have patient cost assistance that I am sure will be widely used to help all CFers afford the drug. But again, compared to hospitalization or even transplant, this drug is much cheaper in the end. It's called pharmacoeconomics.