Surprisingly, you might find it cheaper by flying to Frankfurt, and either connecting to Amsterdam, or catching an ICE to Amsterdam (yea! 180+ km/h in a train!). Alternatively, try flights into Brussels with a Thalys TGV connection to Amsterdam. From AMS, there is relatively easy car hire or direct trains to Amersfoort Schothorst (where the event is).

I suspect it would be worthwhile trying to book as early as possible: also, a block booking from a group of people travelling may also bring the price down somewhat, but you would need to make pretty quick decisions and coordinate it together.