Chicago had a lot of great Brunch places, some of which can handle a larger family group for a very reasonable price.
A few that come to mind as being particularly notable, as well as somewhat special.
Hilary's Urban Eatery - on Division, just east of Milwaukee & Ashland. They have a great brunch buffet on the second floor, as well as a large outside seating area in the back. About $14-15/person with a very wide range of items on the buffet (and a very good menu downstairs if the buffet is not of interest)
Home Cafe - only open on the weekends, just north of the Francisco Brown line El stop. A smaller space, so with a large group you may want to talk with them in advance. The food is seasonal and extremely good. Traditional dishes, but with a bit of a soul food kick, and very well seasoned and expertly prepared and seasoned. During the week catering businesses are run out of the space, so that is another option as well for you.
Around Chicago are many other restaurants with weekend brunches, depending on the size of your group I would suggest that you look for a restaurant with the style of food that you like and talk with them about making arrangements for a group of your size. Most likely you should be able to find a place that for $20 (or less) per person can provide a really excellent weekend brunch.
Shannon