Your data mirrors my own experience. I promoted three different posts and two of three were busts regarding ROI and the new users I did pick up came from engaging with them, not from the promoted post.
I don't think I'm going to participate in the promoted list moving forward until there is some compelling reason to do so.
I can relate to you. I was just about to spend 50$ yesterday, but then seeing people spend 300-400$ only to get back 30$, doesn't look like a good investment. Even the ones on the top 5 spots, they've lost money too.