Apple Logic Pro /LogicExpress Discussion

In my humble opinion:

1.  How serious is the project?
        a.  Music designed and produced for large scale broadcast?
        b.  Uncle Fred and the Garage Band?
       c.  Somewhere in between?

If it's serious I would, as Geoff suggested, send if off to a pro mastering house.  Airshow 
Mastering in Boulder is near me, I might support the local guys (who's credits include Pink 
Floyd, others).  

If it's mostly for fun or just less serious I would "master" it myself with Waveburner 
(included with Logic Studio).  I would also suggest reading Bob Katz' book on mastering.  
Self mastering should have a name of it's own, because when I "master" my song I know 
full well that I am not a trained engineer for legitimate mastering, and that I lack the 
dedicated tools to do the real thing.  That being said, I have read enough about mastering, 
have decent enough ears to recognize when my mastering plugins/settings are making 
the song worse.  I have taken classes and read countless books and magazines.  I have 
listened to demo CDs of "before vs after" from Airshow mastering.

Another benefit to having the song professionally mastered is to have a fresh mind and 
fresh set of ears. 

So it comes down a study on your part of cost vs benefit.   Also in the equation is the 
possibility that you'll try to do it yourself and not be happy with it, and end up getting it 
professionally mastered, or even mixed again. 

I hope this helps!