Fly from Chicago ORD to Portland, Oregon or vice-versa for $105 round-trip on United Airlines. This fare is only available during a small window in late Summer. Flights are available on Tuesday, Wednesday, and Saturdays from August 28 through September 12, 2018. If you can't find the exact dates that work for you, one-way fares are available for $53, and you can build an itinerary around those.
Searching on United, I could find seats departing ORD on Saturday, September 8 and returning from PDX on Tuesday, September 11, 2018. Again you will need to be flexible to book this fare, but it is a solid price for this route.
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