Don't know what prices in June will be, but current price for a litre of white diesel in Holland is about 95 eurocent. I don't think marinas offer discounts for large volumes as I have never refuelled in Holland.
I paid 98 cents 2 days ago. But that was for the car. If your trip is by boat, there'll be the usual 'marina mark-up'. Last year this was anything between €0.15 and €0.25 per litre with no discount for quantity. I'll be filling the boat on Tuesday if you want bang up to date prices.