high CTR and Relevance score will get you cheap CPC.
If your post is not being received well ( low CTR, too spammy), Facebook will scam you. I've been charged as high as 5$ CPC for some non engaging posts. I've also gotten close to 30 Cents US per click, which is not "dirt cheap" but considering i was in a pretty competitive niche, it was not bad at all.
Also too narrow targeting can drive your CPC through the roof, so watch out for that.