The new Office 2013 version is bringing Document protection to the cloud for the first time. There is a service plan in OFF365 / Azure named Windows Azure AD Rights Management (AADRM) . That is a symmetric capability to the on prem AD RMS.
The AADRM is not on by default for your tennant;-> Activate HERE
And enable in Off365:
Site Collection Admins can now enable IRM protection on individual documents libraries : 
Enhanced in Office 2013, the IRM settings UI for a document library is easier to use. library admins can do the following:
- Set access rights, including rights to print, run scripts to enable screen readers, or enable writing on a copy of the document (new to Office 2013)
- Set expiration date (the date after which the document cannot be used
- Control whether documents that do not support IRM protection can be included in the library
- Control whether Office Web Apps can render the documents in the library (new in Office 2013)
- PDF supported by extension download
Quanta Costa?
Free 
This is included in Enterprise plan 3 and 4 and Academic plan 3 and 4
Good Luck protecting those documents