You are right on. I did a similar trial couple of months ago and here are my observations https://steemit.com/steemit/@gokulnk/are-steemit-promotions-dead

One suggestion would be do not spend amounts like $10, $20 and $30 in promotions. It is not worth it. It is pretty much like google search results. Nobody really takes a look at posts after the first page. If you look at the promoted tab now you will see that the first four posts are 100$ plus. So there is no point in outbidding them. But the fifth post is 60$. So I would spend 61$ on promotions and make sure that my post is in the fifth position.

By following this approach I was able to get better ROI on the investment I made for promotions. But I was still not happy and have not spent on promotions after thats.