I've heard both ways as far as benificial goes. My friend had decided she wanted to try natural childbirth and hired a doula. She and her OB actually ended up kicking the doula out of the delivery room! My friend's baby was posterior AND had his head turned to the side. A terrible position for delivery and she was having tons and tons of pain. She was losing control and begging for pain meds but the doula kept telling her "No. You can do it." and telling her that she was giving up and would regret taking the epidural. The OB was losing patience with the situation b/c my friend was becoming hysterical and impossible to calm down. The OB said "do you want an epidural?" and the doula said "No, she doesn't" and my friend says "Yes" and the doula says "no, you don't" and the doc said "please leave!!!!" And gave her the epidural. Which was a good thing because her baby ended up a forcepts delivery after 3 hours of pushing.
IMO a doula can be a good thing IF you are strong enough to assert yourself and she is flexible enough to understand you might change your mind on your birth plan. All sorts of things can come up during labor, and you never know how things will go. It's not "giving up" if you decide to change your mind.
That's why I decided against one. I knew I was going to take the epidural, though, and didn't really need any coaching that my DH or mom couldn't provide, plus, one of my nurses was a close friend. As for expense, I'm not sure, but I've heard they are very expensive. Some hospitals have a complimentary doula service, though, if you request one.
DS born 07/05/2003
DD born 3/24/2005