Depedns on the school. The TKD place I go to, the best I've seen in the US, charges only $25 per month since the instrructos does it on a volunteer-basis (not, that I've been to a lot of different schools). On the other hand, my hometown's community college offers courses for credit and offered Judo, Juijitsu and Aikido while charging $80 for the whole semester.
And as for the other studio that I go to, theyt charge $65 a month and I've seen other schools charge $150 a month as well. There's a wide range of what they charge and usually prices do not concur with the skill with this single exception:
The only warning sign to look out for is -
If a school offers a set plan that guarentees a black belt by the end, do NOT go there - usually they add on that until you get your black belt, you can attend as much classes as you want - what they DO NOT tell you, however, is that once they have your money, they practically throw different ranks at you so you can get your belt, the contract is over, and they can charge more . . . that, my friend, is what we call a "McDojo".
With all that you've said, it's not enough information as to judge whether it's ripping you off or not. $70 per month isn't unusual for a private dojo to charge. They do have to eat and have shelter too, you know. Go to as many different schools as much as you can (even the ones you're not interested in - you'd be suprised), kinda get the idea of what each school is doing (and also what all the schools are doing that's the same), and then decide. This is the exact same thing as shopping around for a car. Until you sign up and hand over your well-earned cash, you're incontrol. Don't let them convince you otherwise.