The OP doesn't say when he's running into the high rates, but I suspect that it may be for the RSNA conference next week. As per usual, the Marriott properties are joining their bretheren in the lodging business in completely ignoring Mayor Daley's repeated pleas for restraint in jacking up room rates during major events.
This isn't to be confused with the local convention bureau and chamber of commerce types' continuing amazement over the fact that they're having an increasingly tough time competing with other cities for convention business.
As for alternative hotels....glg's advice is spot-on.