Ha! This is a good idea. Actually I'd count it by the employee pay/cost being soaked up in the meeting instead of an arbitrary rate. So in your case say the average per hour cost to the company for an employee is £50, and there are 10 people in the meeting for an hour, that meeting cost the company £500. It can keep meeting sizes small as a motivator to keep costs low ;)
(I've seen several $20,000-30,000 "meetings" before that accomplished nothing)
Set a low ball hourly rate - so £50 (depending on where you are and who you're with) so that people suspect the meeting actually costs more.
When it comes to the end of the meeting, you say "Did this meeting produce £376.42 worth of value for the company?"
If the answer is "yes" - you're doing something right. If the answer is "no" - hold a meeting to discuss how to become more productive in meetings.