The cost of promoting your ad depends on the feature, city and category. The prices are set so that the feature remains a premium option for those that decide to purchase them. For example, if the price for Top Ads was set too low, there would be too many Top Ads in that section, thereby your featured ad would have less visibility.
You can find the cost of each feature by going through the posting process for new ads, or by viewing your existing ads in Listings, pressing the three dots and selecting Promote listing.
Please note: prices may change at any time.