The cost of processing user data above the chosen subscription plan is $3 per 1K users for the Basic plan, and $4 per 1K users for the Business plan.
For example, the price for processing 1,002,000 users in the Basic 1000 plan would be $950 + 2 * 3 $ = $956.
The statistics on the unique users processed during the current and previous months is available on the Data usage page. You can find it in the bottom of the left-side menu:
Open the left sidebar and click Data usage at the bottom. On the Data usage screen, you can choose the reporting period, the month you want to see the statistics for.