Cost per click directly depends on your maximum bid, quality score, and Ad Rank as per your competitors. For say your maximum bid is $5 and your competitor bid is $7, so as per your quality score and Ad rank, actual CPC gets effected.
Relevance of the keyword to the ad copy
Relevance of the the keyword to the landing page copy
Relevance of the ad copy to the landing page copy
Historical click-through rate (Google assumes that if your ad is clicked frequently for a particular search query, the user sees it as relevant)