▶ The size of your ad(s): the bigger the ad the more will it be priced.
▶ What publication(s) you use: the more famous a publication the more shall be the cost.
▶ The frequency with which you run the ads: the more number of times the ads are run the individual cost will be low but the overall cost will be more.
▶ What sections of the paper(s) you want your ads.
▶ Whether you use enhancements in your ads.