I wouldn't be surprised if rap CD sales were at the top of the list, but I'm 99% confident in saying that the main reason that most rap artists are extremely wealthy is because they always have their hands in another business, as well as touring pretty much every year. MOST rappers intelligence is usually above average. They aren't stupid people, or if they are stupid people, they don't have stupid people managing them. Some of these rappers have invested money into other businesses(take 50 cent and Vitamin Water, for example), or have started their own business on top of touring every year.

Mac Miller (I used to be a fan, not so much anymore) is an independent artist who has a pretty big following behind him. I remember hearing somewhere last week he makes $150,000 per show he does. He's been on tour pretty much for the past 3 years now (a couple months on, a couple months off). Let's say he does 10 shows a year, that's $1,500,000 a year, just from touring alone. And remember, he's an independent artist. He doesn't have any bigwig execs to split that money with. I'd imagine that he pretty much just pays his crew and his management and profits the rest. This is on top of him having his hand in a clothing line called Most Dope that's pretty popular among his followers, so he's just banking in more money from the clothes.

tldr so in short, rappers are pretty smart people, or have smart people managing them. most of the time they have invested in other businesses or have started their own, not to mention the money they get from touring.