Permanent storage is used only by transfers that are available forever (which don't have an expire date). Transfers which do expire don't fill up permanent storage.
Each Business user gets 100GB of permanent storage for free - that is included in the subscription price. When a user stores permanent data above that limit we apply a flat rate of $0.10 per GB per month. Storage fee is billed monthly.
Example: user stored 140GB (i.e. 40GB above the free-tier limit) for 15 days in a 30-day month. The monthly bill is:
40GB x $0.10 x 15 days / 30 days = $2.00