Here is the TL;DR version of the last post.
Catalonia runs a budget deficit with the rest of Spain. That is disguised because Madrid kicks back a large chunk of Catalonia’s tax revenue in the form of soft loans. In other words, Catalonia has been taking debt owed private investors as it comes due and transforming it into debt owed to the central government. (This far to the tune of €52 billion.) That means that an independent Catalan state would need outside financing if it declared independence as its debt came due, to the tune of €4 billion per year; such financing would be impossible in a contested divorce. Even if Catalonia renounces its share of the national debt, Spain can lock it out of financial markets. The result will be crushing ... and that is before accounting from the trade shock that will come from Catalonia’s hard exit from the European Union.
(Note that tax revenues are lumpier than spending: even a state with a budget surplus needs access to short-term financing to make its payments. Losing access to finance will be crushing.)
Rough numbers for Spain:
Explanation: Spain in 2014 received a net transfer from Catalonia worth 0.8% of GDP. Losing the territory would cause an increase in spending of 0.3% of GDP, since the new smaller country would have to cover the same burden of fixed costs. (Defense, foreign affairs, regulatory authorities, public research, etcetera.) If Catalonia also renounced its share of the national debt, then the rest of Spain would also have to assume an addition 0.6% of GDP in interest costs. The total cost, then is either 1.1% of GDP or 1.7%, depending on what happens to the national debt.
Rough numbers for Catalonia:
Explanation: with independence, the soft loans go away. Catalonia will be in deficit unless it renounces its share of the national debt or allows net taxes to rise substantially. Note, however, that Catalonia will still need access to short-term financing. Also note that these figures make no account for the inevitable trade shock that Catalexit from the E.U. will bring. That trade shock will depress revenues and raise spending, making access to capital even more critical.