Total EU admin costs approx 9 billion, a lot of money but divided across 28 countries? considering they represent 28 countries at the WTO, negotiate FTA's Air safety, Nuclear safety, Customs, rules an regs for trading standards, EHIC ....... if each individual country had to do all this, how much would it cost? and you'd still have all those very generous civil service pensions!
Of course it can be run more efficiently, CAP for starters but despite that we are leaving, farm payments are still be guaranteed for a decade or more.
Prepping for brexit has cost the UK 3 billion already and thats before we get into no-deal territory.