There is currently a Southwest sale going on where the flights are priced based on the distance of the flight at either $49, $89, $119, or $149 each way. This is the type of Southwest sale that is actually pretty good. In fact, I make many of my bookings during these type of sales as other airlines have a tendency to match on routes that otherwise are often much more expensive. You must book by this Thursday for travel from April 3 – June 8.
You can get all the details of the Southwest sale here, but check your preferred airline to see if they are matching as well. You can get some ideas of available fares on other airlines on this Flyertalk thread. My advice is to move sooner rather than later as some flights start disappearing before the official end of the sale. I hope to post more on this sale when I have time to play with it myself, but hopefully this will get you started until then!