Advertised prices include Washington National to STT for $159 each way. (I tried but couldn't find it ... probably you have to fly Tuesday night and come back the following Thursday Monday at midnight or something). Similar deals from Atlanta, Chicago, Providence, and others. I'm sure if you hunt and are flexible, you can save a bunch.
Sale fares must be booked on spiritair.com by 11:59PM ET on October 9, 2005! Travel dates for this sale are Monday through Thursday between October 11, 2005 and March 31, 2006.