Go to cheapflights.com and enter your flight search parameters. Then click on all the search options on the right to find out what different websites can offer you. Chances are many or most will offer the same flights, but at somewhat different prices. Also, the booking fees can vary widely and are a source of hidden costs. Make sure that you know the real total cost, and book the cheapest flight you can find. This way, I found a one-way flight from Detroit to San Diego for April 23rd for $140, which I think is a pretty decent price.
I will buy my return ticket somewhere in southern Washington when I know when I'm going to finish and where I'll be flying to.
Checked baggage often costs extra for domestic flights. Typical costs are $15 to $25 for the first bag, and $20-25 for the second. Carefully consider whether it makes more sense to check your trekking poles, matches, gas stoves, knives, etc., or to mail them to yourself and pick them up near the trailhead. Postage will almost certainly cost less. I'll have to make the decision myself: pay $15 for the checked bag, or send my package to the town of Campo, just 2 miles into the hike from the Mexican border (a quick google search shows that it opens at 8:30 a.m...).