I agree with the Palmer house but expect to pay a bit. Weekend cost me $150/night back in December, normal rack rate was over $300. I had the Hilton at ORD make the reservation and got one night (Thurs) for less than $90.
Also, if you need something downtown you'd better look early as the tourists fill em up.
You can get really cheap rates at ORD hotels and then take the blue line dowtown (about a 35 minute train ride to downtown) There is a Days Inn on Clark and Diversey (Lincoln Park) that used to have cheaper rates. The rooms are small but there is a bus stop right outside to take you towards the loop/downtown area.