I charge 2% per calendar month; so my main customer, @silvergoldbotty gets to use 50K of my steem power and pays me 1000 steem each month for the privilege.
I also have two smaller customers, who each pay 200 steem per month for 10K SP.
I like it because it increases my exposure to the steem price.
They get to upvote their communities and causes without a massive initial outlay and I make out like VISA with zero risk of default.