I think most money is earned int the art/pop section, it seems like most people just use Steemit, like Facebook, for everyday posts. Nothing serious, just pop culture.
I think the promotion feature has negative expectancy, so you probably won't earn more money back than you put in. However it can be used to gain more followers, if you are willing to burn through some capital in order to "climb up the ladder faster". But it's not like an investment, you probably wont earn more back than you put it, unless you are already making money.