I have only seen one bill DaveReidUK. It did not have the reason on it.
To be fair the reason was crew stupidity definitely in that case they had missed 2 FIR boundary frequency swaps and had 123.45 dialled into box 2 out of NL over Belgium and were intercepted over France.
I believe the c word was used quite a few times in the post incident debrief. you pair of , you stupid etc etc
both of them were given a photo copy of the bill which was in the region of 30K euro apparently it would have been more but the two FJ's were up in the air already so they only got billed for 30 mins flight time.