It's not a Brexit divorce bill, it's actually the price of staying in

That some money is going to flow from the UK to the EU during Brexit seems obvious. Quite how much, and quite how much will flow the other way in the split of assets, isn't obvious as yet. But one thing we do think needs to be clarified - this isn't a divorce bill, it's the cost of staying in:

The UK will pay money to the EU as part of the Brexit process, Boris Johnson has conceded, having previously said the EU could “go whistle” over a reported bill of between €60bn and €100bn.

The issue of the “divorce bill” has split the pro-Brexit camp, with some leave supporters demanding that the UK decline to pay a penny, and Johnson told the Commons last month that a demand for €100bn (£92bn) would be extortionate.

We repeat, whatever the bill is isn't the cost of leaving, it's the cost of staying.

For that is in fact the justification being used to demand it. If you Brits had stayed then you would have paid this much into the EU budget. In fact, you've already agreed to pay this much into the EU budget. So, you should pay this much into the EU budget.

That is how it is all being justified, demanded even. If you stay you will pay this. Thus this amount is the cost of staying, not leaving. The leaving part is just making it more obvious that this would be the cost of staying.

What concerns is not the size of the bill, but the way that it is being presented. 

Another way to think of this is that the £50 billion, whatever it is, is a sunk cost. And if we stay then more such bills will arrive in the future, if we leave none will. That doesn't change anything about reality of course but it might change the way people think about it, getting the situation correct.