It's hard to comparison shop dojos.
Different skill levels of instructors, different opportunities for classes (can I attend one class a week at a set time or do I have my choice of 6 different classes) different class sizes and what national/international organizations the dojo belongs to (which can effect insurance and what tournaments you can compete in) all effect pricing. Be careful not to end up comparing apples to oranges.
You also need to watch out for other fees. Many dojos have belt fees that you have to pay everytime you test. Some of these dojos have MANY belts and do things like yellow belt- orange tip, then yellow belt - purple tip, etc. with each belt having a test and a fee. Some of these are doing this as a way to make more money off the student. Others are doing it because if they don't have advancements all the time, then there students don't see progress and quit. They charge to offset the hassle of doing the extra advancements. On the other extreme I've seen some dojos that charge you a non-refundable flat rate up front that covers every class until you reach black belt no matter how long it takes. Of course, if you drop out (as many do) you don't get your money back.
I've taken classes for free (either because the instructor did it as a hobby and for the love of the art or because I helped out at the dojo) and I've taken classes for cheap (an aikido club that used the fees to pay the rent for the room) and I've paid $50 a week for 1/2 hour of private instruction.
I know I haven't really answered your question, but I hope this helps.