Ballpark figure is around 60bn Euros, which is around 12% of the EUs liabilities until 2020. However, it's not really debt as such. It's more along the lines of agreed payments to cover EU projects.

One of the contentious parts is the 'cohesion fund' which is used to pay for infrastructure projects in less developed EU nations.

The long and the short of it is that, after Brexit, there is no way to realistically force the UK to pay up.

It's a useful bargaining chip, unless the EU is prepared to make some concessions and step away from their sabre-rattling, punishment rhetoric there nothing to stop the UK from essentially saying, "Balls to you then. We're not paying you shit. We're off to be friends with America, India, Australia, Pakistan, China, Canada, Japan and Taiwan...But you can stay here with your refugees. Enjoy!"

Of course, if the EU do step away from this rhetoric then more countries might be tempted to leave. It'll be interesting to see how they play it.

Regardless, the UK should come out of this just fine. I think that the president will want to 'reward' a nation escaping from the forces of globalisation so a nice trade deal with the US might be on the table. In that case the UK would come out of this very well indeed.