We paid $23 on the first day when we parked at Navy Pier. It is usually a flat rate for 24hours and anything else is gets broken down and it is quite expensive!
We did notice some discount parking areas throughout the city, but with an entrance before 7am and exit by 7pm (times do vary among parking garages) and some prices we saw were about $15 for the day.
another pointer, if you are taking the Architectural River tour (at the corner of Wagner and Michigan) you can park at 111 Wagner parking garage and your parking ticket will be validated by the tour booth giving you a nice discount!!
I also have to mention that many of the hotels charge a separate daily rate just for parking, so be aware!
If you can use local transit, I think it will be more economical.


