First off, per the other answers so far, €50 billion (or €100) is by no means peanuts.
More importantly, this article gives, I think, a reasonably level-headed breakdown of what the Brexit bill is about and what's at stake in practice.
A large chunk of the money being asked for by the European commission was pledged by David Cameron, when he was prime minister, to the long-term EU budget for the period of 2014 to 2020.
But the EU, with the UK as a member, has also agreed to pay for programmes that will be implemented in the years 2019 to 2025 and possibly beyond, including road, rail and investment projects.
Brussels further wants the UK to pay up to cover the costs of pension promises to officials and MEPs, and other long-term EU liabilities. If the EU’s loans go wrong, for example, the European commission wants the UK to play its part in covering the losses. These are known as contingent liabilities.
There are a few numbers a bit further down in the article:
Some commission briefings have pointed to an estimated gross bill of about €100bn, including:
- €86.4bn to honour commitments the UK made as a member state, including about €20bn to cover 2019/2020, the last year of the seven-year budget to which the UK was a willing signatory.
- €11.5bn of contingent liabilities, in case organisations or states, such as Ireland or Ukraine, fail to repay their loans.
- €1.7bn in development funding pledges to help countries in Africa, the Caribbean and the Pacific.
The EU doesn’t necessarily want all the money in one sum, however. It is open to regular payments over a period of years.
Moreover, regardless of the precise figure for the gross bill, various factors will bring the net bill down.
... with this addendum after some discussion:
The EU has so far insisted that the UK cannot offset its share of European Union assets, such as buildings, or indeed the commission’s generous wine cellar, from the bill. However, there are cracks appearing in the united front and, privately, European commission officials admit that member states may be being a bit unreasonable.
At any rate, the commonly quoted gross bill is closer to €100 billion rather than the €50 billion you're citing without source. The article puts the net bill at around €75 billion. It's anyone's guess what it'll be in practice.
Either way, what the bill is about should shed light on the EU's insistence. It's actually money that the UK already has committed to pay. Either outright, or to cover losses in case of defaults by third parties, plus some foreign aid and pensions for UK citizens that served as MEPs and EU officials.
The UK arguably has some room to negotiate the bill down, if only by insisting to offset the bill by the value of assets it lets go in the process. The pension of its own citizens who worked for the EU will hopefully be a no brainer.
The rest seems like negotiation leverage in that, per the above-mentioned article, nothing obliges the UK to foot the bill after leaving the EU. However, the UK is not in a good position to renege on payments it already has committed to. The EU's interest is to send a clear signal that leaving the EU is not a panacea; both the EC and the EP have been making pretty clear statements earlier this summer that they intend to send that message.