For visitors to Portland, the cheapest way to arrive and leave from PDX (Portland International Airport) is via the MAX light rail system. There are four color-coded lines: Blue, Red, Yellow and Green.
MAX connects the airport with downtown Portland; the west side silicon forest area of Hillsboro; the eastside suburb of Gresham; Delta Park and Expo Center to the north; plus the southwest suburb of Clackamas, including the big shopping mall.
Trains run every 15 minutes for most of the day. Trains may be less frequent in the late evenings.
The ticket price varies, but currently tops out at $2.35 for a two-hour adult ticket to $4.75 for an adult *all zone* one-day pass. http://trimet.org/fares/index.htm
Once in Portland, MAX is a great way to get around without a car and there is no charge to use MAX or the Streetcar for travel within downtown Portland, Rose Quarter or the Lloyd District. This area is called the Free Rail Zone. http://trimet.org/fares/freerailzone.htm
There is a current special with Radio Cab from downtown Portland to PDX for $26 or from PDX for $28.50 for a cab. This is a good deal for a flat rate cab vs using a shuttle. www.radiocab.net