What information does a target cost-per-acquisition (CPA) bid strategy need in order to find the optimal cost-per-click (CPC) bid for an ad each time it’s eligible to appear?
Manual bid changes
Historical conversion data
Using historical information about your campaign, Conversion Optimizer automatically finds the optimal equivalent CPC bid for your ad each time it’s eligible to appear. You still pay per click, but you no longer need to adjust your bids manually to reach your conversion goals.
Read more here: https://support.google.com/partners/answer/2471188?hl=en