Fly from Philly to Charleston for $160 round-trip, including all taxes, on Continental. This fare is valid for travel on Tuesdays and Wednesdays, with a 330-day travel period. Tickets require a 21-day advance purchase. No minimum stay.
You'll find lots of similarly priced fares to/from Philadelphia today (look for the $118 pre-tax fares) on both Continental and United. Same rules apply, with plenty of Tuesdays & Wednesdays available for summer travel.