I booked two return trips from Heathrow to the Dam for 2007, one in June and one in November at peak times of the day and different days (going out on a Friday & Tuesday, back on a Monday and Saturday respectively) and paid the princely sum of £0 for the seat each time; just paid the taxes - total £56 return each trip. You can't get cheaper than zero for the seat
fly me:
http://www.flybmi.com/bmi/en-gb/index.aspx
or me:
http://www.bmibaby.com/bmibaby/html/en/splash.htm
Edit - think i'll book another trip as well