Do you mean Portland?
If so, check Southwest.com. If you book the flight far enough in advance, a lot of times it's really cheap, especially right now when they're having all kinds of promotions going on. You can sometimes get a flight for as cheap as $53 one way, if you book it at least a month in advance.
You could also try priceline.com They have the "name your own price" feature, and they normally have good deals on airfare as well.
at 12:40 PM on Sep. 5, 2009