I had doulas for both my births. I would recommend a doula to anyone; they are worth their weight in gold.
Here is what I got out of my doulas.
1. A long prenatal meeting (some do 2) where we spent a long time with DH discussing our hopes and plans for the birth and agreeing on how we would work together.
2. Tons of phone support - when I had an issue come up I could talk it over with someone well-informed who suggested questions to ask and resources to help me make the right decisions in dealing with the care providers (scare-mongering OBs in birth #1 - they say the baby might be transverse! they say the baby might be too small! etc.)
3. Company at home during early labor; I always wait as long as I can to go to the birth center or hospital - I labor in my own space and go about my life. But I still liked having someone there with me for support (see below) and if something unusual came up, to be an "expert" on what was happening.
4. Assistance with decisions during labor - what to do when water leaked before cx started? (nothing, just wait) how to react when cx did start but very lightly and late at night? (have a glass of wine, go to sleep!) when to call the midwives? (in the morning when cx are more serious) when to leave for the hospital? etc.
5. Physical support - massage, suggesting different positions, helping with physical relaxation - for the tough parts of labor
6. Emotional support - my doula sister used amazing "non-coaching" while I pushed - no counting, no cheerleading, no yelling - just whispering about how fantastically awesome and strong and amazing I was. I'll never forget how much it helped me keep pushing.
7. Husband support. Could your DH do all the above? Well, maybe some. But he hasn't been at 10-50 other births so he's no expert. Also, he's probably pretty freaked out. And, if he is busy massaging your back, he can't simultaneously time your contractions. Will he also remember to remind you to pee and keep drinking water and maybe try walking around now? And if he does, he'll probably forget to eat or drink anything himself. So the doula will take care of him too - and be an extra pair of hands.
Why do doulas cost "so much"? Well you are paying for at least one prenatal and one postnatal visit - sometimes more - up to 4 hours plus. And you get their availability for phone/email advice. And you are "reserving" their time. Many doulas take only 2-3 clients a month to ensure they will be present for your birth. They don't go on vacation if your due date is near, and they cancel all plans if you go into labor. Being "on call" this way for weeks is the big downside of the doula life. And, births can be overnight, on weekends, 18 hours long, etc. If you do the math, paying $500-900 for all that is reasonable and with only 2-3 clients a month the take-home pay isn't much.
Some have had luck getting some insurance compensation for doulas and its worth asking - but don't count on it. Still, I advise doing it anyway.