like I said, you can still trade, just not on such favourable terms. It means American or Chinese goods will be cheaper abroad compared to ours, European goods will be dearer here. I don't have any recent figures on just how much the EU 'costs', but it's not all one sided - billions out, nothing in. There's an upside as well.
Plus historically, the aim of the EU - which has been successful - has been to stop Europeans going to war with each other. We may not have fed millions of pounds into Europe in the 40s, but we lost millions of lives.
Not just WW2 but the cold war. Boot out Romania, say, and what happens? Russia moves in with offers of cheap oil and government handouts, and pretty soon a buffer state against Putin changes sides and a little bit of iron curtain goes up again. Is it worth paying to stop this happening? I would say so, others might not.