Parking downtown chicago is hell.
Any chance of getting it in a Chicago Suburb? Any place near Rt 290 would be fine, since thats the only freeway and its just a 20-30 min drive to Chicago on it during non-rush hour from the NW Suburbs like Schaumburg or Hoffman Estates or Elk Grove.
Not to mention you wouldn't be subjecting people to paying the horribly overpriced Parking Meters in the downtown area. I am all for free and plentiful parking.
Wizard World Chicago is at the convention center in Rosemont. It's not actually in Chicago. There's plenty of parking there but it's not gonna be free. I think it's like $10-$12 a day and that's on top of the $50-$60 it will cost you for a one day ticket to WW.