I've noticed these types of questions popup in every business forum that I've visited that instead of responding the same thing over, I've decided to create website specifically for Open Source business applications. I haven't officially launched the site yet, but I already started posting some common responses there instead of retyping it again. You may want to check the following link for a brief review that I wrote about my experiences with low cost Open Source CRM solutions which include billing and invoicing:

http://theopenzone.org/viewtopic.php?t=5