This is beyond the realm of Aperture out-of-the-box since it's not a compositing editor like Adobe Photoshop.
If Adobe Photoshop is beyond your budget, Mac OS X's Preview app has this "Instant Alpha" ability built-in. Read more about it here: http://lifehacker.com/318083/do-more-with-preview-in-leopard
Also, try giving Pixelmator a try. It's relatively cheap. You can read more about doing this in Pixelmator here: http://www.pixelmator.com/learn/2009/04/quickly-remove-unwanted-background/
Hope this helps!