Is that relevant when we talk about GDP ? Even if little money flows from Barcelona to the central government (I don't know if this is or is not the case, since I don't live in Spain), that shouldn't make a difference, since "GDP measures the monetary value of final goods and services - that is, those that are bought by the final user - produced in a country in a given period of time (say a quarter or a year)."[1]
There are 16 billion per year going from Catalonia to Spain without return.
But not only that, the centralisation of government powers and agencies have created a problem where many big companies have to be in Madrid even if most their operations are in other parts of the country. That makes their taxes to be paid in Madrid instead of e.g. Valencia, Bilbao or Barcelona. The amount of taxes lost because of this is HUGE and much worse than the 16 billion per year above.
An independence would bring back 100% of the former and a good chunk of the latter. Hopefully that would help Catalonia grow at the pace its economy can and then it should be able to contribute back to Europe much more than now.
Contributions have to be sustainable.
Edit: This comment is now at -1. Please argue instead of downvoting. You're acting against HN's rules.
CNI (Centro Nacional de Inteligencia), like CIA in Spain, is working a lot to hide this comments and arguments. You can see a lot of comments which are the same speech as Partido Popular.
In the Catalan media they appear in waves, polluting all the comments and making the discussion unbearable.
They are also holding cyberatacks to the Catalan News Agency so we cannot inform outside.
They have also closed aerial space, so drones cannot fly and take pictures of massive demonstrations tomorrow if we cannot vote.
Maybe you were thinking about taxes ?
[1] "http://www.imf.org/external/pubs/ft/fandd/basics/gdp.htm"