The difference here is between how children react and how grown ups react. In the grown up world your words along with the SPIRIT of their meaning underpins your actions. in this case all the parties agreed on splitting the costs, not retro actively changing the rules to suit themselves.
Well, the dinner metaphor breaks down here. It's almost certain that the access to the EU workforce and trade markets have benefited the UK much more than the pocket change it pays into the EU; but it's hard to put an exact number on those things.