RDS seems to be doing just fine with a large markup over EC2. Why would the managed version of a service be lower priced than the same infrastructure being managed?
Amazon has more possiblities to run a really huge cluster and balance the different load peaks of the search nodes. The whole business model of AWS is based on this... but you're right as well, price isn't everything and comfort worth money. And yes, I love RDS as well, especially Aurora... it's a great deal!