Richie,
Most of mine ran no more than 5Euro, showers about 0.50Euro extra. Here is a sitefinder; click on the location, then More Info, and rates should appear:
http://www.natuurkampeerterreinen.nl/en/campsite-locations/site-finder/Of course, there's the 15Euro annual membership fee, but that is not bad given the savings.
The NL doesn't have many hills, but they have wind in spades. The trick is to go with the prevailing, realizing that can change at a moment's notice. I got caught in some Beaufort 8-10 gusts for a time and had trouble keeping the bike on the ground. Rain along the Coast can be a given, so pack rain gear. Lots and lots of places to shop and resupply with food and water, or stop at the numerous herring and frites-and-mayo stands if you need cheap food quickly.
A really nice route would entail flying into Amsterdam, training north to Friesland, then heading south along the North Atlantic (north-to-south; I've gone the length both ways, but most of the prevailing is from the north) to Amsterdam or perhaps Leiden (where some of my past English relatives stopped for a couple years on the way to America to meet the Irish half of the family...along with those from LeHavre, France and Germany and Hungary...), then either further south to Zeeland (Vlissingen, Middleburg) and/or east to the Green Heart (Hoge Veluwe, the big national park in the middle with bits of African Savannah melded with blooming heide/hather and forest with lots and lots of animals you can view from big photographic blinds), then on east to Maastricht, a very nice city. The hills (Yes, the NL has a few, and they are respectable) begin just east of there in Vaals, culminating in the Vaalsburg and Drieponte Park where The NL, BE, and Germany all abut borders.
You'd have a nice, flat ride though all but the Vaalserberg, camping opportunites -- particularly in the East -- are abundant, and the Natuurkampeerterreinen campings would fill in the gaps. I love the Weert, NL area especially (one of our Forum members hails from there) and Zeeland is "Old Dutch" with lots of farming and open spaces. Attitudes range from surprisingly conservative on the SW North Atlantic coast to nude beaches near a military base in Zeeland as you approach the Schelde. Google has blanked out the base from all its maps and -- unfortunately -- a bit immediately around it, but it is still easy to navigate.
Generally, people in the south and east of The Netherlands seemed to eat more heartily of a non-seafood diet, being a bit further from the sea. More pork and that love snert compared to fish and my beloved raw pickled herring on those little red-and-white plaid paper trays with diced onions. I'm getting hungry thinking about it!
Thinking...if you did an end-to-end from N-S along the North Atlantic, then threw a left at Zeeland, you could have a nice ride all the way to Brussels and still avoid hills. If you didn't mind some small rollers, you could continue to one of my favorite places in Belgium's "bread basket" and see Jodoigne. Not much there, really, but a really nice mix of farmlands and smaller hills. Oteppe is nice, too. So many great places in that part of the world!
All the best,
Dan.