No it's not.
there are 3 main factors at work here:
1) the money that we owe the EU each week
2) The rebate that was negotiated
3) money coming directly back into the UK from EU schemes
there are also a bunch of other benefits which may/may not be benefits / costs - but lets look at the direct parts first.
the figure for 1) is 350 million (or it was - it actually varies due to the economic output of the country, but we can use it to start).
the rebate means that figure is reduced - i think by about 70.
IMPORTANTLY, it is not the case that we sent 350 and get 70 back - we only send 280.
we then get additional money back to spend on specific areas, as we pay into an EU budget for a program and then (sometimes) some of that money is spent in the UK. Estimates put this at about 50 (i think)
my basic understanding of how accounting works suggests that in fact, we would budget to spend the 280, not the 350. Counting like Boris does means that you are effectively spending that money twice.
To paraphrase Tim Harford:
"You walk into a shop to buy a TV with a price of £350. But there is a deal on at the moment, meaning that the TV is only £280 and you can get 50 of netflix vouchers. the "true" asking price may be £350, but you only need to spend £280 to get it. you then get an additional benefit of the netflix vouchers but have to spend that on netflix. You decide not to buy the TV and spend the money on something else. How much money do you have to spend on the other thing? Answer: £280, not 350".