Your account will be charged only for setting a task.
The cost can be calculated on the Pricing page.
Setting Google Search By Image SERP Tasks
Google Search By Image SERP API provides up to top 100 search engine results based on the image you specified. These results are specific to the selected location (see the List of Locations) and language (see the List of Languages) settings.
There are two different priorities that stand for the relative speed of task execution: normal and high.
