Given that our net contributions to the EU are about £10bn a year, and we won’t be paying this anymore, about 4 years.
I don’t think we will be expected to give £40bn upfront though. If we give £10bn a year for 4 years it’s the same as now.
But we won’t be getting the subsidies/grants that we currently do from the EU and will have to replace at least some of those from our own budget, so it’s not entirely straightforward.
Of course that doesn’t take into account the potential extra costs of new customs, immigration etc arrangements and potential slower economic growth that predicted if we stayed in.
The subsidies and grants are taken into account in the 10bn current contributions, since it's a net figure.
Some part of the £40bn is not intended to be clawed back as it is payment for liabilities - projects, pensions etc
We will get money back from the European Bank but over a long period.
I don't take much notice of slower economic growth predictions, since they are made by economists that got it very wrong in the months post the Brexit referendum.
Bloody BBC!