Changes to the Google-WhatsApp agreement could increase the cost of using WhatsApp for some customers.

Backups to WhatsApp can be stored indefinitely on Google Drive.
Unfortunately, it appears that Google's relationship may be terminated in the near future. WABetaInfo, a reliable source of information regarding WhatsApp's upcoming features, has revealed lines of code that refer to Google Drive's limitations.

It's too early to determine whether such changes will occur, but it's also not worth doubting. Furthermore, neither the sum to be charged for data storage nor the payment procedure are disclosed (one-time, monthly, yearly, and so on).