I think Marche has a pretty big room -- I'm certain I did an event there for over 50 people.
You might want to rethink your seating -- maybe if you compromised on the set up you might end up still having a good stimulating discussion and yet not have to spend $100 per person. U shape isn't typical in most restaurants, I think.
Grand Lux Cafe has a pretty big room and I think I"ve seen it set up banquet style which might work for you.
One thing about Marche -- the company that owns them has several other properties and they've always been really good to work with -- they may be able to suggest something else.
Good luck!
Grand Lux Cafe
600 N. Michigan Ave.
Chicago
Marche
833 W Randolph St
Chicago, IL 60607
(312) 226-8399
(312) 226-4169
marche-chicago.com