BahUllah:
Propably cheapest and most convenient way would be by Eurolines busses -> www.eurolines.com (www.eurolines.nl for netherlands, www.eurolines.fr for france and www.eurolines.es for spain)
With Ryanair you end up paying for extra fees and transport to/from airports far away and you most likely pay at least as much as with Eurolines.
With Eurolines busses you can depart and arrive from / to city centers. Travel by night and you'll save with accomodation.
example prices: Amsterdam - Paris 36 euros & Paris - Barcelona 75 euros
Edited by: Od