From 2009 July 1 the vliegtax (an environmental supplement on flight tickets) will be scrapped. The tax - which was introduced on 2008 July 1 has lead to a supplement of 11.25 Euros on a European flight and 45 Euros on long distance flights. The introduction of the tax has been blamed for many travellers preferring to use airports outside of the Netherlands.
According to Stijn Bronzwaer's blog on NRC Next, the rise in flight ticket through Schiphol is not just the fault of the vliegtax. The Schiphol Airport itself raised the cost of flights. However, from 2009 April 1 the airport will be lowering its flight tariffs. This is to maintain the airport's position in the international travel market.
Web resource: NRC Next - Stijn Bronzwaer