Lunch between visits to Art Institute and Millenium Park?
The restaurant in the Modern Wing of the Art Institute is called Terzo Piano, a reference to its location on the third floor. It has Italian cuisine and it is very good indeed; it's from Tony Mantuano, the founding chef-owner of Spiaggia. It is open most days for lunch, but only a few days for dinner. As for affordability, you can view the lunch menu with prices on its website at www.terzopianochicago.com
Other good choices in that immediate area include Pizano's on Madison (deep-dish pizza, www.pizanoschicago.com ), Heaven on Seven on Wabash (cajun/creole, www.heavenonseven.com ), the Gage (gastropub, www.thegagechicago.com ), Henri (French bistro, www.henrichicago.com ), and Park Grill (American, www.parkgrillchicago.com ). None of these is terribly expensive at lunchtime - there aren't many places that are - but if you're trying to keep the cost down, Pizano's is probably the least expensive of these.