From your example of Calais, we already invest in the port in order to tackle the illegal immigrant problem towards the UK...On page 1 where I said:
That, for me, could connect the reinvestment.
For example, if France wanted to improve the port at Calais we could contribute if we thought it would benefit the UK.
But that requires a reciprocal agreement...
One that we are about to lose!
And we used to be a beneficiary of the EU when we were the 'sick man of Europe, but now we contribute to other less well off EU economies...
And in return get free access to their markets thus benefitting our economy...
That we are also about to lose!