Hi - I've never used a doula, but I totally recommend the Gentle Birth Method (book and CDs) by Gowri Motha. Much cheaper and gave me two, brilliant, quick, natural births.
It was especially helpful with the first, as it's all so unknown. It gives you the confidence that you can do it. It's mostly about using your mind to control pain (and the fear of pain). Birth is painful but if you practise the right mental techniques, you can massively lessen the pain you feel. When you feel pain and feel afraid, your body tenses up, which can make it worse and labour longer. This book, and CD, teach you how to manage that process to have a quicker and less painful labour.
BTW - should warn you there's loads of very strict sounding stuff around alternative therapies... I just ignored that (could be expensive and time consuming) and focused on the mental visualisation part and giving up wheat. Seemed to work for me! Good luck!
I can't afford one but we are practicing hypnobirthing which is meant to help with a natural birth! We bought a book and did some research (couldn't afford the classes) and I feel good about it so far.
I have a fb pregnancy blog and I wrote a whole post about hypnobirthing, take a look if you want xx