Flying anywhere in Europe from the US can cost close to $1000, but right now tickets are going for less than half that price. You can fly to Dublin, Ireland, which also happens to offer tons of RyanAir flights all over Europe, for less than $400 round trip. Basically, you can go wherever you want for under $500.
Leaving from Philadelphia, Air Canada is offering flights to Dublin starting at just $394, and there's pretty decent availability. Here's a sample of the low fares seen on Google Flights for the month of October, but keep in mind that The Flight Deal is reporting availability spanning September 2016 - March 2017.