Your account will be charged only for setting a task.
The cost can be calculated on the Pricing page.
Setting Google Ads Search SERP Tasks
 
Google Ads Search provides information on ads that are run by advertisers on Google Ads. Information is based on the Ads Transparency platform and adapted for the convenience of DataForSEO users. 
Google Ads Advertisers SERP API provides top search engine results specific to the specified location (see the List of Locations) and other specified parameters.
There are two different priorities that stand for the relative speed of task execution: normal and high.
Note: your account will be billed per each SERP containing up to 40 results.