To calculate CTC eCPA, you take the total media spend for a given time period divided by the click-though-conversions (CTC) for that same time period.
EXAMPLE: Mark is running a display campaign for Apple’s new iPhone X. The goal or KPI of the campaign is to drive online iPhone X sales from Apple.com
Mark spent $500 in media yesterday running the Apple campaign and it received 25 conversion, of which 15 were post-view and the rest (10), were post-click.
Mark’s CTC eCPA for yesterday Apple’s iPhone X campaign was $50 (500/10)