Facebook Ads’ performance is not consistent. One month you can get a 55 cents cost per click, and another month 80 cents, targeting the same audience, with the same ads.
There are many factors affecting performance: other advertisers bidding for the same audience, users getting tired of your ads, seasonality related to your market.
You will look silly if you project a 30 cents CPC and then you get a $1 CPC; even worse, you will have to explain why the delta to your superiors or clients. Even if you project a $1 CPC and then you get a 10 cents CPC, you will still look like you don’t know what you are doing.
You want to get as close as possible to a stable and predictable performance. Facebook is a black box: if you change an ad’s creative you don’t know how that would impact its CPC.
All you can do is to plan ahead.
What’s the max CPC you can afford? What’s the max amount of orders your company can handle for a given month? What’s happens if the campaign doesn’t provide the min amount of orders for a given week?
You don’t have control over the Facebook’s auction, nor you can promise a definite CPC or CPA number. What you can do is to set performance goals, and if the campaign doesn’t meet them, react accordingly.