A little bit of browsing can help you find the smartest and cheapest methods to travel from the airport to downtown Chicago.

Chicago O'Hare airport ground Transportation

http://www.airwise.com/airports/us/ohare/transport.html

Shuttles leave the O'Hare airport every 15 minutes. For sub urban service and hotel service, the shuttle service starts at 5:30am and 4:00am respectively. The charges charged by the cabs are based on the mileage and time basis, so the rates vary accordingly. It is highly recommended that you reserve well in advance. They are required only if you opt for special services like going to a residential area and for your return to the airport.

Chicago Transportation

http://www.chicagotraveler.com/chicago_transportation.htm

A round trip transfer

http://www.partner.viator.com/en/2819/tours/Chicago/Chicago-Airport-Roundtrip-Transfer/d673-3178ORDAPTRND

O'Hare Airport: The cost for the shuttle service is quite nominal. It costs $27 for single person to move to downtown or from downtown to O'Hare airport. If you opt for round trip, you can save $5, as round trip costs only $49. Similarly, if two people travel together they charge $19 per person for one way and $35 per person for round trip. For more than three people, they charge $14 per person for one way and $27 for round trip.

Midway Airport: It costs $22 for one person for one-way trip to travel to downtown or from downtown from Midway airport. For a round trip, you need to pay only $37, saving $7. Similarly, there are other offers for two person and three persons, which you can see from their website.

Chicago transportation - 4 simple ways to get from the airport to downtown

http://ezinearticles.com/?Chicago-Transportation—4-Simple-Ways-to-Get-From-the-Airport-to-Downtown&id=4369972