£350 million was the figure bandied about pre-Referendum. Take a read of this joey - interesting:
https://fullfact.org/europe/our-eu-membership-fee-55-million/
At one point it says, after rebates etc., that it's £250 million a week. Still seems to me that we are thowing good money after bad (as the phrase goes).
In the interests of being fair and driving the middle way between what has been stated, and what it actually seems to look like, here's what that article says just over half-way down:
"In 2015, the UK's ‘public sector receipts’ amounted to £4.5 billion
So overall we paid in £8.5 billion more than we got back, or £23 million a day."
Now £23 million a day works out at £161 million per week.
£161 million per week works out at £8,372,000,000 per year.
I would venture to suggest that is a hell of a lot of cabbage to be throwing to the EU buckshee every year... Now some might say that we get this, that & the other from the EU - wrong, that has already been taken into account when arriving at the base figure of (just under by my calculation) £8.5 billion per year.