I'm sure it will be an arm and leg if it has anything to with the city of Chicago.
Try the Morton Arboretum in Lisle, (I believe). It is very scenic an beautiful there.
It is $33,000 to rent Buckingham Fountain. Spaces near Millennium Park seem to be more reasonable.
You can check out the prices for Chicago Park District rentals at: http://www.cpdit01.com/resources/permits.special-event-venues/
It is at least $20K to rent the space, plus tenting costs. An expensive option unless you have a large budget.
Ann Marie Arzt
Go to http://www.cpdit01.com/resources/permits.special-event-venues/ for more park district options. There are some very nice options that aren't quite as expensive as Buckingham Fountain. But if you have the loot burning a hole in your pocket, knock yourself out! I'll wait patiently for my invitation. :)
Click on each tool to see a sample.