I've seen 90% compression on databases before, I would not expect a number like that across all databases all the time. And as Bill Taleda said, don't compress if you're storing binary objects in your DB as you'll be wasting time and getting no compression out of it.

But as Brandie said, there is no "expected compression", it depends entirely on the makeup of the database. Lots of text fields or empty space: higher compression. Binary objects: lower compression. It's kind of like trying to zip a jpeg: it's already compressed, so you're not going to gain very much. So tailor your backup options according to how your databases are built and used. If you need to store binary objects, put them in a different file group, back it up separately, and don't use compression on it.