Well, with some places of business, yes you can ask them for the difference:
http://www.bestbuy.com/site/Electron...at107400050005
But in the airline business, only Southwest (that I am aware of) offers a penalty-free credit when the fare drops if you buy non-refundable tickets.
A number of years ago NWA used to offer a $25 (IIRC) refare. It was a good deal^