American Airlines has round-trip flights from San Antonio (SAT) to Philadelphia (PHL) starting at $134, NONSTOP. $67 each-way. Flights March to January. A lot of weekends available. Holiday Blackouts apply. Origin/Destination reversible.
Note: Carry-on bag and personal item is allowed for free. You can avoid paying the seat selection fee by accepting the free randomly assigned seat at the gate.
Note: Frontier seems to be offering a few flights for a little less than this, but those will usually cost more due to Frontier bag fees.