I would suggest to read

http://msdn.microsoft.com/en-us/library/bb964719.aspx

you will discover

"•Whether the database is compressed.

If the database is compressed, compressing backups might not reduce their size by much, if at all."

Maybe I have missed something with my poor understanding of the English language, but , according to me , it means that you must not hope a significant gain of space with a compressed backup on a compressed database.

I have done several tests with a Developer 2008 R2 edition 2 years ago and I have obtained a decrease of 1% which is ridiculous.

I recognize that I have only a Windows 7 desktop with "classical" disks. Maybe , on a "super" Windows Server 2012 with marvelous disks , the gain could be more visible, but if you have to pay twice ( or more ) to gain 20% of space , I am not sure that it is valuable solution ( less space is good but , if my remembrance is good , what a waste of time especially in case of disaster )

Have all a nice day

PS : I hope you will excuse my poor written English