I think a lot will depend on the doula community in your area. I live in Phoenix, AZ and have now had 2 experiences with hiring doulas.
With my first birth, I paid $500 when the avg range was between $450-$800.
This time around, we cannot come close to paying $500 due to unforeseen circumstances, but I didn't want to give up on my goal of having a doula for this birth. Sooo, although I can't comment on the difference in ability or service level of the doulas (I'm due in March), I can tell you that I am anticipating having 2 doulas present for the birth - one is charging $108 and she will get the "credit" for the birth so she can become certified and the other is volunteering for free eventhough she can't use the birth toward her certification. But I LOVE both of the ladies and feel that if the doula community where you live is like it is here, you should be able to find someone to help you even if you can't pay the "standard" rate.
This is how I went about it: I first went online to dona.org and did a search for doulas in my area. I then sent out an email asking for pricing info/availabililty and was up front with the fact that I would need lower cost services or some kind of payment plan.
I then sent out a similar email to a local ap mom's group through yahoo groups.
From there I received all kinds of emails ranging from reduced fees to offers for payment/barter plans, to additional names of women who are certifying, etc. and I went from there narrowing down the responses, setting up interviews and finally selecting two(!) doulas. I found that every doula I talked to was more than willing to work with me in some way, or was eager to suggest people they knew, etc. No one ever made me feel bad for wanting a doula, but not having the money for someone more "established."
Anyway, good luck in your search
**And just as an aside, I do plan on writing recommendation letters, creating a birth cd with images, etc for both of them and also will be giving them each a gift for their services after the birth. I figure that even though I don't have a lot of cash, I can help them build their businesses with letters and images that show them "in the trenches" that they can share with other moms-to-be. I also do copywriting and have offered to help them with their marketing materials in the future if they are ever in need. So something like that might be nice if you work with someone for a lower fee.**