This is the airport parking map: http://www.flychicago.com/ohare/tofr...s/economy.aspx. The orange line is the transit which runs quite close to the Hilton, the $40 daily rate applies to the lots around the Hilton, which is off the top of this map.

ETA: This map shows the main parking and the Hilton: http://www.flychicago.com/ohare/tofr...maindaily.aspx. I note from there that the transit is being replaced by busses until next summer.