Well with 1 SBD you get what you have paid for and that is, the upvote from the @promoted account.
If you check the latest posts in the "promote" tab you can see that you really have to pay a lot to even be on the first page. It's worth just because of that upvote. You will not get that much more engagement by this, because you didn't get into the "hot" or "trending" tab. :)