Well, according to [0], Aetna's revenue in Q3 2018 was $15.5 billion - so, extrapolating, their yearly revenue was something on the order of 60 billion dollars, of which that 500 million you mention is just 0.8%. Also, some of that $500M is in stock, not cash.
--
https://www.modernhealthcare.com/article/20181030/NEWS/18103...