How about that, Google Apps allows buying additional storage, seems even for the Apps part:
https://www.google.com/accounts/PurchaseStorage
- 20 GB ($5.00 USD per year)
- 80 GB ($20.00 USD per year)
- 200 GB ($50.00 USD per year)
- 400 GB ($100.00 USD per year)
- 1 TB ($256.00 USD per year)
