Round trip nonstop fare from New York City (EWR or JFK) flying into Amsterdam (AMS) can be found as low as $406 with wide availability from February to early June.
The airlines could be either Lufthansa (may be sold by United but operator will be Lufthansa) or KLM (sold under Delta) depending on your travel date and airport.
Availability chart assuming 7 nights stay.
Book your trip on Priceline, Travelocity, Cheaptickets, Cheap-o-air
