Or rather, the attention cost. If Facebook has printed your ad 1,000 times and it cost you $10, it doesn’t mean anything from the optimization point of view. It is 0.01 cents per impression, what a bargain, let’s get more. There is no obvious action to be taken other than keep spending money on the channel.
What really matters is how many people you’ve reached. If your audience is 1M users, and you’ve reached 500k at $5k, and you want to reach the rest of your audience, you have two options: A) spend another $5k or B) refine your audience.
And when you start refining your audience is when things get interesting. In Facebook, you can get a breakdown by region, and see the cost of reaching 1,000 per region. If for whatever reason, a region is not relevant to your business, you can remove it, and relocate the money to those regions that you care about. Or if -again for any reason- a region is too expensive, you can remove it as well, and find the one that is relevant to your business and cost-effective.
Below is a sample report you can get from the Facebook Ads Reporting tool.