The Eurostar website here:
http://www.eurostar.com/UK/uk/leisure/destinations.jsp
is showing returns to Amsterdam, from 8th to 11th August, 2 adults, at £379 total, which is in line with what you were getting. Obviously, there are cheaper deals if you fly, but I'm guessing Eurostar is part of the "special occasion" package. if so, the next question, obviously, is - does it have to be Amsterdam? If it's the canal/architecture thing that's important, why not try Brugge or Gent in Belgium?
If it's got to be Amsterdam AND Eurostar, it looks like you'll just have to pay the asking price

If I can help any more, just ask.