I haven't flown DC-Germany, but in general certain airports are cheaper than others. It can depend on the airline, the time of year, the specific route, fees, etc. If price is a major concern, I'd try all sorts of of combos of DC (Dulles, Reagan, and BWI if it's feasible) to any German city on your itinerary. Also, I bet September is cheaper than June.
I like kayak.com for this, but I'm sure there are other sites.
Not Germany related, but a few weird flight pricing anecdotes:
-I'm going to Spain in late March. Flying on March 27 (Thursday) was several hundred dollars cheaper than flying on April 3 (also a Thursday). Apparently, the seasonal pricing changes hit April 1, and by leaving a week earlier I save a ton.
-I used to live in Buffalo. Flying Buffalo to London was always $300 more than Toronto (2 hours away). This one makes some sense (big airport vs smaller), but Toronto seems to be cheaper than other big airports to London.