Delta and Air France have Philadelphia to Paris at $431 to $441 round trip from Oct 20 to May 10, but it looks like these low fares primarily work for trips of 4 or 5 nights in Paris or fare rises to $700s. This fare even works for short trips at Christmas. There are most dates from November to March and fewer dates in April and May. Here are two examples for Christmas and May travel. Oneworld and Star Alliance fares are in $700s at time of posting (1:30pm California time).
Note: Fares looked like they just changed while I was writing this and now longer trip dates are available than 4 or 5 days. You may want to book fast if these fares interest you.
Delta Airlines Philadelphia PHL – New York JFK – Paris CDG – New York JFK – Philadelphia PHL
$435.96 round trip Wed Dec 21 – Tue Dec 27 (6-day trip gives 5 nights in Paris).
Delta Airlines Philadelphia PHL – New York JFK – Paris CDG – New York JFK – Philadelphia PHL
$435.96 round trip Sat April 1 – Mon April 10
