Well, if you're a highly successful artist you will make a lot of money and will become very wealthy. Last I heard Taylor Swift's net worth was estimated to be ~150 million and that was before her highly successful "1989" album release. A couple years ago she bought a roughly $18,000,000 house in the same town as my parent's summer home in Watch Hill (Westerly), Rhode Island. I can tell you she's not as popular around town as she is nationally. Paul McCartney, one of the most successful artists of all time, is supposedly worth $660m.
But yes, the artists usually get a very small proportion of the action. That said, if they sell millions of albums, it ends up being a lot of money. For not so successful artists, they won't make a whole lot of money.
As for "The Case of the Ex" (by the way a very random choice), I think the song meaning is quite literal as it's a story. It's about a guy who starts talking to his ex girlfriend and Mya (the new girlfriend) finds out it and is angry/jealous. I don't think there is much room for interpretation with this one. I'm not sure if you had other ideas or if the music video portrays some other story. This is a mainstream pop song that I do not see as containing any "deep" message needing to be deciphered. It's popular because it's catchy and people can relate to the storyline.
