Ok my workings outr are as follows:
Hemel to Leiden is about a 600 mile round trip...(Excluding ferry) so looking about £70 Petrol.
Ferry £25 again!
Hotel - Holiday Inn Leiden E200 for 2 nights in a family room... (assuming drive up Sat am and home Monday, rather than a cross europe-dash Sunday for a ferry!)
So a weekend away for about £280 (+beer/food) and again probably best split between two people per car.