I paid $328 on my last trip, this last May, via American. I flew out of Portland. This was not a red-eye flight.
I think that Fontier was showing up as slightly cheaper, but I didn't like the schedule for some reason. I don't remember exactly why I choose not to use them.
I paid $356 in December 2004, and few American West, and took the red-eye flight one of the two ways. For this trip I flew out of Eugene rather than Portland. I think the price out of Portland was about $50 cheaper. Usually the difference is closer to $100.
For both trips, I booked at least a month ahead, for the December trip I think I booked about 3 months ahead.
The airline with the best deal varies.
I tend to use expedia or orbitz to compare, and then go directly to the website of the airlines that show up cheapest.
Most airlines offer their best deals on their own website.
I don't think I have ever paid over $450.
If airfares are close (not much difference) I choose by schedule.
It is an advantage to choose an airline with more flights. That way if a connection is missed, there is a greater chance the airline can accomodate you quickly.
I like to arrive by 6PM, and don't like to leave before 1PM.