I've done a quick check around and it's hard to find decent figures, but as I suspected it looks like nobody is quite telling the truth here. Norway probably pays in about half as much as we do, but doesn't get anything back. Whereas we're full participants in the EU, so obviously receive CAP payments, regional funding and other stuff. Thus their net contribution is about 90% of ours.
I've seen Remain people argue that they pay 90% of what we pay and don't get our rebate - which is a huge distortion of the facts as that figure is being compared to our net contribution (payments less receipts).
Their analysis probably slightly understates Norway's net contribution and overstates ours, but it's bugger-all difference in the grand scheme of things.
The calcs are done on GDP per head on a pro-rata basis for each are of funding you choose to join in on. Net of whatever cash you get back.
There's an argument that we could negotiated to pay in less and still get Single Market access. But a lot of this money is cash we want to be spending. Regional funding to help boost the economies of Greece and Eastern Europe is something we'd have done anyway, after the Cold War. It was us that were one of the strongest voices calling for the EU to allow the ex Warsaw Pact countries in. And for the accession process with Turkey too, and there's a good argument that Turkey has only taken a turn to the authoritarian because it was clear that France and Germany (and others) weren't going to let them in. So they've been busily reversing all the rule-of-law reforms that they'd been implementing under EU pressure for the last 2 decades. I'd say the EU's great success is that the Eastern European countires have managed to integrate into the system and build working democracies. Compared to the appalling state of politics in the ex-Soviet countries that didn't get to join.
Anyway we'd be well out of the Common Agricultural Policy if we left, which makes up about a third of the EU budget, and our contribution. And is partly the reason for our rebate, in that our farmers in general get fewer subsidies from it.
But we'd want to continue infrastucture funding in Eastern Europe, I imagine we'd stay in the EU science and space programmes so contribute and receive from both. So I'd expect our net contribution to drop a bit, but not a huge amount. We'd lose the rebate, but then also lose the reason for it, and since Blair gave away a chunk of that in exchange for promised reform of the CAP that never happened, plus our lower GDP per head than Norway, I'd expect our net contrubution to drop a bit below theirs.