CPA - Cost per Acquisition - Formula: Cost / Acquisitions. Calculation of avg. amount of budget needed to invest for 1 new acquisition. There are many types of acquisition like signups, purchases, first time purchases etc. It's important to choose one type and use that as the main metric for CPA calculations.
ROAS - Return on Ad spend - Formula: Revenue Generated by ads / costs of ads * 100. This calculation shows how much amount of money you are generating for each 1 ad spend. To be profitable the ROAS should be above 100%. If it's below 100% that means that for each 1 ad spend there's a loss.