Depending on your account's plan configuration you can be charged for disk space based on:
Charges for Individual Resource Quotas
When you increase:
Read more about it in Online Prices section.
Charges for Summary Disk Usage
If by the end of the billing month your average summary disk usage exceeds summary disk usage limit, you are charged usage (overlimit) fees.
If you increase your summary disk usage limit, every month you will be charged recurrent fee which is usually less than usage fees.