Eurocontrol charges are pretty steep, especially in/from the UK. They are also calculated using the square root of the aircraft weight; so Ryanair, using smaller, short-haul aircraft pay more per tonne than BA using a jumbo for long haul (for example).
Almost right, you have to divide the weight by 50 first though and then square root it.
This would give a weight factor of 1.18 for a 737-800 and a factor of 2.81 for a 747-400.
The full charging formula can be found here:
Eurocontrol Charge calculation