I think that the birth experience one is seeking will help determine what type of birth class to attend.
I cannot speak for other women, but when I was pregnant with my first child, I planned on having a natural childbirth at Special Beginnings Birth Center (now closed unfortunately).
When a friend suggested I look into Bradley Birth method, I called the Bradley instructor and spoke with her in depth about what the class offered. It was a 12-week course aimed at helping women and their partners understand the physiology of birth while also encouraging the right amount of emotional, physical and mental support required during the marathon of birth. The curriculum even calls for a trial "labor practice" that allows pregnant women to imagine what each stage of labor will be like. After attending the first class, I was hooked.
At the end of the 12 weeks, all of us in the class were sad to see it end. We felt completely knowledgeable and capable of having the birth that we sought. We knew EXACTLY what we were in for. I cannot say that for most of my friends who have had their children in hospitals (unfortunate, but true).
I also took the 4 weeks of birthing classes offered by the birthing center--they were fine, but only touched the surface and did not prepare me for the mental task that lie ahead of me.
In retrospect, I'm not sure if I would have been prepared for everything that a natural birth required of me had I not taken the Bradley Birth classes. I remember thinking in the beginning that it seemed like a lot of money, but after I had a beautiful labor and birth, I remember thinking I would have gladly paid three times as much to get that same experience.
I've heard many, many women say that they plan to have a natural birth but have done NO preparations in advance. Unfortunately, many of these women end up with too many interventions (i.e. epidurals, pitocin, etc...) and ultimately end in C-section.
So, you just need to decide what your birth wishes are and then see them through. If you are planning a natural birth, money will soon take a back seat to your positive birth experience.
Happy Birthing!