The figures are pretty simple - in 2017 Britain contributed £13 billion to the EU budget (the third largest contributing country) - in 2017 Britain received £4 billion back from the EU = a net loss to the UK of £9 billion.
In such a narrow view you indeed lose and is this 4 billion received back the rebate + EU subsidies? The EU is however by far the largest trading partner of the UK and the potential damage done there far outweighs this 9 billion gain and cannot be compensated for by trading more with the US or Brazil, if only because of geography.... What seems to be happening is that Britain still wants a good relationship with Europe, which is indeed beneficial to both, but they seem to have forgotten they can't really influence EU policy once they are outside.
Clearly the EU has twice as much to loose as the UK does.
Without its third largest contributor, the EU in its current form is financially unsustainable. As it appears to be too bureaucratic and self-important to reform, implosion is inevitable.
Do you think the EU is incapable of performing a roughly 5% budget cut, that is a pretty grim view indeed. If Brexit has united a people, it is the Europeans in the fact that they will let the British feel the real costs. I have hardly ever seen such a united EU as in the Brexit negotiations.