Simon,
There's a good and extensive choice of camp-sites in the area around 's-Hertogenbosch and Waalwijk in Noord Brabant.
Suggest you take a look at the Dutch ACSI review web-site - and filter candidates by price (very close to Efteling you may be looking at Euro 40 a night - dropping to Euro 20 just a few km away). The English language version of ACSI cna be found at www.eurocampings.co.uk
I work about 90 minutes drive from Efteling - so if there is anything specific I may be able to help.
Oh - and don't forget to look in at "Camping Sport De-Wit" in Schijndel - just a few minutes from Efteling - De-Wit is the largest camping shop in Europe and cheap too (for camping stuff) www.de-wit.nl
Robert