Right been re-visiting this.
Ferry on 11th July return 14th (Fri-Mon) is £29 Dover-Dunkirk, then a 180mile blast up various E-roads to Leiden, Hotel is about £200 for the weekend too (Ibis)
So all in all if there's two in the car you're looking at about £150 each before spends.
Not a massive amount for a Long weekend in Europe with a show thrown in too.
Oh and Am-dam's only a 30 minute tram ride away!