Cost of Facebook Advertising totally depends on various factors such as
Competition in category
Time of posting
type of facebook ad
Here i would like to give description about every factor that impacts the cost of facebook advertising. These factors are as follows-
Competition  in Category
Some categories are more competitive while some are less. The bidding price automatically becomes high in the highly competitive category.
Time of Posting
Time of posting impacts a lot in facebook advertising. The cost of faceboo advertising also depends on time of posting. The cost automatically increases during the ideal time of posting. Because every advertiser like to post during that time. They compete each other to find the best slot.
Type of Facebook Ad
Facebook ask to set a goal while advertising. It may vary such as to increase clicks, website visitor, conversion etc. The cost per lead is automatically gets high in compassion to cost per lead.
Quality of Ad
The average cost of facebook advertising campaign depends on the quality of ad. High quality ads attract more visitors and engagement. Facebook gives preference to engaging content. Cost of facebook advertising get down if you write engaging content.