An analogy:
I pay a certain amount of council tax, of which £X is spent emptying my rubbish bins at the curbside and in the street. I could look at it as buying £X of services from Veolia. If I bought the same services myself, how much would I pay. Am I getting good value for money?
Or I could look at the overall benefit of living in a society where there isn't rubbish lining the street wherever I walk. I feel like the people who say "we make a net contribution to the EU" look at the arrangement as a cash transaction without taking a holistic view of the resulting environment.
Of course, I'm sure some economist could come along and put a price on living in a society without rubbish lining the street, but to most people that's not a monetarily quantifiable thing. So I'm sure someone could put a price on all of the below points, to varying extents. But I'm fairly certain when people talk about "net contribution" they are not doing this.
So, that acknowledged, intangible benefits of being in the EU I feel, off the top of my head:
- common standards and lack of trade barriers that let companies in the UK make things and sell them abroad, paying tax in the process
- common standards and lack of trade barriers that let me buy something made anywhere in the EU
- ease of international travel for leisure and cultural exchange, including immigration, aeroplane safety and standards
- living in a country where I can eat cheaper summer fruit because migrant work can flow
- (very specific) I'm friends with instrument makers who do business in the UK and the continent. The import/export tariff situation (as things are currently looking) means that they won't be able to do business
- living in a country where researchers and academics can travel in the EU to do whatever they need to do
- not having to think about travelling to meet with colleagues and vice versa
- contributing to a bloc which enforces environmental standards that align with what I consider to be Good Things (although the CAP is arguable)
- ditto animal welfare
- ditto human rights
- ERASMUS
You can't eat money. My life would be incalculably less rich for having grown up in an insular country without many of the above. Maybe I'd be a few pounds better off. How much happiness could I buy with that?
I'm not saying that the EU is perfect by any means. It could do with a bit of reform, which the UK could have helped with.