Each call, regardless of the amount of information returned, will count as one transaction.
The first 500 transaction per month are included for free with the purchase of the Wordpress/API add-on.
The next 3000 transactions per month are $0.01 per transaction.
Any transactions above this are $0.001 per transaction.
The API tab of the worker profiles will show a history of transaction counts. If using multiple profiles, you'll have to manually add them up.
The transactions count is reset the first of each month and you are billed for last month's transactions on your normal bill date.