Now is very early to be booking for September; fares will probably drop between now and then. (About two months out is usually a good time to book, though these things are not very predictable and if you see a fare you are happy with you should get it.) Krakow is pretty well-served by several European low-cost carriers, so chances are you'll want to find a cheap city to get to from DC elsewhere in Europe (you can use http://www.kayak.com/buzz to help you find one) and then use a low-cost carrier to get from there to Krakow (use skyscanner.net to help you find one of these).