Chicago is a big convention town so at times when you think you should be able to reserve a hotel with no problem (like the middle of February when the temps are below zero) and are finding really high rates, there's probably a big convention in town.
Before booking that airfare, you might have a look at this website to see what is in town when you will be visiting, don't worry about the 500 person conventions but if you see 40,000 attendees listed, you might want to rethink your travel plans. Not only will the hotels be jacking up the prices but it will be harder to get reservations at restaurants as well.
In addition to conventions, there are two other events that bring in lots of out of town visitors making rates go up, the Chicago marathon which is held at the beginning of October and Lollapalooza which is held in August.
Th Congree Hotel was at one time looked upon as a leader in the hotel industry of Chicago. Unfortunatly since then, they have done everything that they can in order to rid its hotels of any and all of its unions, because of this we the tourist suffers when staying there.
For the past 7+ years you have not been able to go into the hotel without being bombarded with an angry mob of people telling you not to patranise the hotel. If you dare continue walking into the building you will be cursed at and spit on by the people in the crowed.
If you are looking to come here and enjoy a hassel free environment, avoid this place like theres no tomorrow. The picketing unions often times send people posing as guests into the hotel to shut the elevotrs down as well as cause other kinds of problems in the hotel. I had a friend that was staying on the tenth floor for a weekend and the elevators were shut down shortly after he checked into his room. The hotel never even comped his room for all of his troubles.
Dont believe me, look at the web site dedicated to the troubles that the hotel has brought upon itself.