You can park for around EUR 35 for a weekend through smartparkingschiphol.com which is a sort of comparison service for all parking opportunities nearby. For under 7 days' worth of parking, it recommends Service Parking, Sky Park and Quick Parking. (The website is in Dutch, as I suppose it's aimed at locals, but as Dutch is the closest related foreign language to English, it is fairly straightforward.)
Alternatively, park your car in a nearby town or village, and take the excellent, regular and cheap train to the airport.
Hope it works out. Have a good trip!