I will give you what experience I have had, which isn't as broad as some on this board......but here goes:
If you are training with a school that offers contracts/agreements, you are generally looking at anywhere from $70-100 per month for classes. You are also most likely looking at a year contract, but the school may offer six-month contracts as well, or even month-to-month. Your monthly payment will usually break down to something like two or three classes a week, and may include a private class per month (like a one-on-one with an instructor). Most places will offer private lessons in a range of $30-40 per half hour, but your mileage may vary.
If you find a school that is offering contracts like this, and you feel good about it, check to see what other costs are going to be levied: i.e., do they charge for testing? Do they charge for belts? Do you need to buy additional gear over a certain period of time? These kinds of questions will be important, because there are schools out there that will nickel-and-dime you to death.
There are also schools out there that don't require contracts, and let you pay month-to-month, or even class-to-class. It depends on the school, the location, and so on.
Best rule of thumb that I can give you is to look around. Ask questions, and do the math. Find a place that teaches what you want to learn, and that you feel comfortable with. As long as that base is covered, the money thing will be secondary.
Good luck to you in the Windy City!!
Peace--