Here's what it says on the SWA website...
DALLAS, Dec. 11 /PRNewswire-FirstCall/ -- Southwest whetted bargain travelers' appetites today by revealing that its Philadelphia service will begin May 9, 2004, with daily nonstop service to Chicago Midway, Las Vegas, Orlando, Phoenix, Providence, and Tampa Bay. The accompanying fares and direct and connecting service to other cities nationwide will be announced later.
..."Announcing our entry into Philadelphia was just the appetizer, and the destinations are the entree," said Joyce Rogge, Southwest's senior vice president of marketing. "Just wait until you see our fares -- you'll want to save room for dessert!"
In other words you'll have to wait until they release the rest of May's schedule your air. I'm sure it will be about the same as BWI's fares, just so you have an idea. Hope this helps!