The estimation gives you your best & worst case scenario with my campaigns. It helps you plan your campaign strategy.
There are multiple reasons why the estimation is broad.
1. Your final CPI is based on Facebook's algorithm which is real-time & takes into account other advertisers who are competing for the same spot as your ad. Hence, it's not possible to predict an accurate CPAI without running the campaign.
2. There is extensive testing involved in getting the maximum out of the campaign. This is done on in order to get the most out of your $'s. These factor in the costs that can eventually be achieved. Hence, without running the ads it's impossible to predict a precise cost per action.
3. Estimations are based on previous experiences of working on similar campaigns
4. It's not possible to accurately predict the cost without running the ads as it's unique for every promotion.
5. Different ad placements have different final costs. The estimation is also sent keeping this in mind.
REASONS FOR THE ESTIMATION
1. Number of competitors
2. Ease of targeting on Facebook
3. Previous experiences of working on similar businesses
4. Current prevalent CPA on Facebook & under my management.
5. Misc. factors like company credibility, audience activity, content etc.