How is the price for additional users calculated with the per-user model?

Monthly subscription:

When you purchase a monthly YouTrack subscription, you choose the maximum number of users that can be activated in YouTrack in the upcoming month, for example, 100 users. We charge you at the end of each billing month according to the peak number of active YouTrack users in the system, which we track on a daily basis. Your total cost will then be calculated based on the lowest monthly price available for your maximum number of users.

For example, if you have 99 active users and add one more user on the last day of the month, your per-user price for this month will change from $4.40 (for 99 users) to $4.39 (for 100 users). You will then pay for 100 users for this month.

In total for the particular month: If you add the hundredth user at any point during the month, you will be billed

              $4.39 * 100 users = $439.00

Instead of:

              $4.40 * 99 users = $435.60

Annual subscription:

At the beginning of the year, you purchase a certain number of users (and save 16% compared to a monthly subscription). Throughout the year you may add more users, and YouTrack tracks your maximum number of users each month. At the end of the year, we will additionally charge you for the total number of extra users added each month, according to the monthly pricing. 

For example, you have 500 users in your annual subscription. Without upgrading your subscription, you may exceed this number by 50% during the year. Imagine you activated 200 additional users in one month of the year. At the end of your billing year, you’ll get an invoice for those 200 additional users based on the monthly price, but with an additional discount, too: 200 users with monthly pricing are discounted to $4.19 instead of $4.40, so your additional cost will be 200 users * $4.19 for that month.