Those sums aren't some arbitrarily thought up "exit fees"
They are, actually:
1) The number being cited keeps changing (upwards)
2) When asked how they calculated those figures, and what the legal basis was, the Commission said it wasn't going to tell you.
There is a useful analysis of what's going on here:
https://www.cer.org.uk/sites/default/files/pb_barker_brexit_...