If I had to choose, then for the cheapest it would have to be Dover Dunkirk with Norfolk Line. Just checked, and provided you are willing to travel between 10pm and 2am, it can be as little as £12 each way car + 2.
From Dunkirk Ferry Terminal to Amsterdam is < > 215mls. and, apart from the first 4miles or so to the A16, all motorway.
Ferries are every even hour for both journeys and take approx. 2 hours, so if for instance you travel with the 2359hrs.Dover / Dunkirk you will be in Dunkirk at 0300hrs local time. Give yourself an easy 4 hr. drive and you could be in Amsterdam for 7am.. This way you would miss the 'rush hour' traffic black spots of Antwerp and Utrecht.