Spirit Airlines is having a sale with fares ranging from $2 to $113 each way. Travel dates vary depending on route, and the lowest fares are for travel Tue/Wed/Sat. Check the site for details. This sale ends tonight, midnight ET.
As is often the case with these Spirit sales, it's a crap shoot whether you'll be able to find the cheapest deals in both directions. For example, they list Atlantic City to Detroit for $2 each way, but we were only able to find that fare from AC to Detroit; coming back the cheapest available appears to be $74 for a total of $76 RT. Still very cheap for that route, but hardly $4.