This can change drastically from moment to moment. Facebook is guessing the CTR of your ads and weighing it against the average ad CTR. This is no different from other popular ad systems on other networks. It starts at the average CTR in the marketplace, then gives a CPC estimate and uses that to establish a ballpark CPM. The CPC price adjusts as your ad history gathers more data. No data (on a new ad) means an almost certainly inaccurate estimate, so plan on running the ad for a day or so, then adjusting your bid at regular intervals.