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You can promote at any cost you like. The more you pay your post will show prior to others. If you can't afford as much as a 1$ for promotion, post in less popular tags. With very small promotion cost like 0.001 SBD your post will maybe be the only one in the promoted tab, or if there are others, the competition is not that big.

So if I promote for 0.28 SBD my post(as I just did) it is not going to be seen by almost nobody.

Depends on which tags you promoted.

My post is the only one promoted in #bwphotocontest but I still dont see a sign of any effect

The Promoted tab is there, you just need to click on it. Evey tag has its own Promoted tab. When people see this tag they can click on the promoted tab and see your​ post. This is the effect.