Wow! Thats expensive Maarten!
The good news for you is that you missed an Airline! Most Online Ticket Searches like Expedia and Cheapflights.com exclude low cost carriers like Easyjet, Ryanair and BMI Baby. I have flown to Amsterdam from Birmingham on BMI Baby before, the low cost version of British Midland.
I just checked their website and they do an outbound flight Friday 1st Dec. from Amsterdam at 1800 Returning on Monday 4th Dec. at 17:35 for 46.62 Euros. (Including Taxes) If you want to fly Saturday and return Sunday it works out at a 46.13 Euros. There are 2 flights a day to/from Amsterdam to Birmingham with BMI Baby except Saturday which is only one flight.
Check out the website at
http://www.bmibaby.comHope this helps, I am sure it may make your decision a little easier now! I would say you had better book early though as there are a lot of Exhibitors coming over from the Netherlands!