I would imagine that's because it's such a massively complex matter that it's impossible to put a figure on it. So why would they even comment?
"No, it'll be less" "No, it'll be more"
There is no benefit to anyone by saying either of these.
Also, from what I've seen and heard so far from our government and politicians - the bland soundbites like 'brexit means brexit' and 'red white and blue brexit', and the shaky ground when they've tried to get more complex, be it ex-pats/migrants or trade - that I can quite believe Juncker is speaking the truth when he says that May and her government are deluded and don't really understand the scale of what they're getting in to. It doesn't seem like posturing or bullyboy, it seems a pretty accurate description.