It cost $10 to promote it. However I get a $5 payout. I'm not sure why. I promote my post so they can get attention. My post that I don't promote generally don't receive any attention, so promoted post seem to be good at getting attention on my post and building my followers. I see it as an investment to get more followers. But since I promote so many of my post, I seem to get a good return from the promote bot.