"The claim that the UK sends £350 million per week to the EU is wrong."
http://www.itv.com/news/2016-04-12/finding-the-facts-the-truth-behind-referendum-claims-on-the-eu-membership-fee/
"EU membership does come at a financial cost. The UK pays more into the EU budget than it gets back. But it's not £350 million a week. The UK’s discount, or rebate, reduces what we would otherwise be liable to pay.
In 2015, we paid the EU an estimated £13 billion, or £250 million a week."
http://www.telegraph.co.uk/news/2016/04/28/eu-referendum-fact-checking-the-big-claims/
"....campaigners all too frequently use the higher figure, despite the fact that it is wrong (and some of those making the claim know this full well). For example, the often heard claim that the UK sends “£350 million per week to the EU” is based on this incorrect measure. The amount that the Treasury remits to Brussels, and thus the direct cost to UK taxpayers, always has the rebate deducted before payment is made. Because of this, the correct figure, taking account of the rebate, should be around £280 million per week (based on 2014 data)."
http://www.independent.co.uk/voices/how-much-does-membership-of-the-eu-actually-cost-the-uk-a6972976.html