The cost of independence would be massive. An independent Catalan government would have to install costly new infrastructure to implement independence. The creation of a Spanish-Catalan border could potentially be economically harmful to Catalonia, as one-third of their exports are to other regions in Spain. Catalonia would also face a large amount of debt. Their fair share of total Spanish debt once they split, is estimated to be around 72.2 billion euros.
Upon independence, Catalonia would no longer be a European Union (EU) nation. It would have to renegotiate trade tariffs with all EU member countries and the prospect of rejoining the EU. Many companies will seek to move their headquarters out of Catalonia. All this may result in a drop in tax revenue and a rise in unemployment.
The tax deficit is not unfair to Catalonia. Tax money is never evenly distributed, and those with the broadest backs carry the most weight. This structure is normal in most countries. Many regions of Spain have even bigger deficits. The exaggeration of the disparity between money-in/money-out of the region is often for political gain.