Not only that, the figure of £350 million is a downright lie. If an actual figure could be gleaned at all from the plethora of financial wheelings, dealings, subsidies, tariffs and rebates that characterises our relationship with the EU, it's more like £160 million. Google this for a breakdown:

"Boris Johnson’s £350m claim is devious and bogus. Here’s why"
(John Lichfield, Guardian, 18 Sept).

As he points out, the "true" figure would be far more likely to be seen as an acceptable cost by people weighing up the pros and cons of our membership. The article also puts into perspective what the numbers relating to our contribution and what the EU spends actually mean percentage-wise when set beside our GDP. They are surprisingly small.