Yes. But not for POP accounts.
The default size of a full exchange mailbox is 5GB, you can then buy additional storage blocks in 5GB lots for $10 each per month.
So a 5GB mailbox will have the default cost of $12, a 10GB mailbox would cost $22 per month. You can upgrade and downgrade during the month but the billing will charge for any upgrades at the price of the upgrade relative to the number of days left in the month.