SW will give you a credit good for use for up to one year. (I think the year starts from the date of your original purchase, but can't really recall.) Some folks have reported getting credits to thier credit card, but as you note they are "nonrefundable" so I wouldn't count on that.
CO will give you an adjustment, BUT they charge $100 for each ticket change so it only works if the price decrease is more then $100. Most of the other major airlines also have change fees that vary from $25 to $100.
However, all of this is subject to change... As you noted, it says "nonrefundable" and your agreement with the airline does not always allow for price adjustments.