If anything because they'll have to rewrite thousands of laws and procedures that came from the EU times, and deal with the caos that that will cause. That only will cost so much money (and pain) that it would justify staying in the EU.
Then, of course there are companies that have left and that will leave, highly-skilled workflow for London that will be harder to get, etc.
They have to negotiate with the EU so that they're not completely cut out of commerce, with nothing to offer in exchange and therefore no negotiating power.
It's old people in the country that have decided this, based on their ignorance on the matter and lies spread before the vote. Funny enough, it's them who mainly benefit from European aid.
Pretty sad, really (I'm Italian, just talking based on what I've read and a handful of friends who live in London).