Here is how you work it out.
European Union Finances 2015: statement on the 2015 EU Budget and measures to counter fraud and financial mismanagement
This shows the UK contributions to the EU budget.
It says in 2105 the gross payment was £17.7 billion. Which includes the Gross National Income payment ( depends how rich a country is) £13 billion, plus VAT = £2.3 billion and a thing called TOR £2.4 billion.
TOR - Traditional Own Resources payments - which is another word for custom duties/tariffs imposed on goods imported into the EU- TOR payments at 75 per cent to the EU. The remaining 25 per cent is retained by the UK to cover the costs of administering collection on behalf of the EU. This amounts £2.4 billion to the EU and the UK keeps £800 million. As this is paid by those exporting into the EU/UK it is not an expense on the UK treasury.
That means the figure is £17.7 - £3.2 (TOR payment)= £14.5 billion includes VAT payment.
The UK gets a rebate - "the estimated value of the UK’s rebate in 2015 is €5.6 billion (£4.4 billion)" £14.5 billion - £4.4 billion = £10.1 billion.
"UK public sector receipts in 2015, mainly from the European Agricultural Guarantee (EAGF), European Agricultural Fund for Rural Development (EAFRD) and the Social and Regional Development Funds, are expected to be around £4.4 billion. The majority of these receipts will either be paid to, or used in support of, the private sector but are channelled through government departments or agencies".
£10.1 billion minus this £4.4 = £5.7 billion.
"The EU makes some payments directly to the private sector, for example to carry out research activities. These payments do not appear in the public sector’s accounts. It is estimated that in 2013, these receipts were worth £1.4 billion. These payments are not included in Tables 3.A or 3.C-F, which provide data on public sector receipts only.
So that is £5.7 billion minus £1.4 billion = £4.3 billion.
The UK could decide to reduce the VAT or it could add it to is coffers which would mean that the money saved by not being in the EU is £4.3 billion plus £2.5 billion VAT = £6.8 billion per year.