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 the rest of Spain. Catalonia would also be saddled with 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 an EU nation and would potentially have to renegotiate trade tariffs with all EU member countries. 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, those with the broadest backs carry the most weight, this is normal in most countries. Many regions of Spain have an even bigger deficits. The disparity between money-in/money-out of the region has been massively exaggerated for political gain.