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The cost can be calculated on the Pricing page.
Competitors Domain (Legacy)
Note that the structure of DataForSEO Labs API requests and responses was updated on 2022-03-19. However, we will continue supporting the legacy version documented here. You can review the documentation of the new version by this link.
 
This endpoint will provide you with a full overview of ranking and traffic data of the competitor domains from organic and paid search. In addition to that, you will get the metrics specific to the keywords both competitor domains and your domain rank for within the same SERP.