Ever since Boris Johnson said Theresa May's Brexit plan was 'crazy', politicians have been dividing into camps over how to solve the problem of the movement of goods, services, and people across the border after Brexit.
What it means: Next month, the House of Commons is supposed to vote on the model the UK will put forward to the EU for how to make this happen.
Brexiteers have this idea they're calling 'max fac', which is supposed to be a super streamlined way of Britain managing its own border system, with 'new technology' which no-one's yet really defined.
But Jon Thompson, who runs HMRC, says that from what we know so far, this model seems totally unaffordable. It'd add a cost of £32.50 to each and every customs declaration, plus another £4-7bn worth of form-filling bureaucracy, which totals about £20bn in extra costs to business... which, incidentally, is double the budget we were paying into the EU in the first place.
Honestly, when politicians start saying things like "between 4 and 7 BILLION pounds" (if only we could have a £3bn 'give or take' window in our budgets...) it becomes next to impossible to make sense of where they're plucking these numbers from.
What's for certain though is that this news puts a serious downer on the Brexiteers' chances of convincing the House of Commons that theirs is the most efficient way forward for Brexit, considering its the MPs who'll have to break the news to constituents that they're adding a pretty big chunk of money to their operating costs.