I just plugged into Kayak using multi-city option
first leg: IAD to AMS on 5/21
second leg: CDG to IAD on 6/14
The least expensive flight I see is $907 on Icelandair with one connection each way (through Expedia, a bit more on Icelandair website).
The least expensive non-stop flight I see is either KLM/Air France or United(both legs) for $1671.
I also typically fly out of IAD and often travel in May. What I've noticed is that prices generally go up several hundred dollars in mid-May. I've found that leaving prior to about May 10th usually lowers the cost. I usually watch flight prices for a while to see if prices go down and I rarely buy before February. However, you never know if prices will go down or up. Unfortunately, over the last couple of years prices have been pretty high and I haven't seen any great bargains.
The Icelandair flight is a good price for that time of year, but unfortunately it requires a connection. They do charge extra for food, but it is pretty easy to buy something in the airport to take on the plane with you. I flew Icelandair this year because they were substantially less expensive than any of the other flights. I normally prefer to fly non-stop, but this year I sacrificed the non-stop in order to save about $400.