The Saudis are running huge deficits. According to articles I've read, Saudi Arabia needs oil priced somewhere between $106 and $115 for economic equilibrium. With oil around $45 per barrel, compounded by increased military spending (Yemen, ISIS), they've burned through more than $62B in foreign currency reserves and have issued $27B in government bonds—the country's first-ever issuance.
In the wake of Arab Spring, the king has greatly increased social spending to quell a restless population that has 29% unemployment for those in the 16-29 YO cohort. 3/5 of the country's population is under 30.
The kingdom should be able to survive for three to five years at current oil price levels—though some prognosticators are now predicting oil at $20/Bbl. Of course deficits will continue to grow and eventually rebuilding reserves will be all the more difficult. But if the kingdom does collapse, what does it mean for the Middle East? And for OPEC? And for the world-at-large?
The irony in all this is that the Saudis are significant contributors to their own economic problems: they've lead OPEC in upping production in an effort to drive prices lower and economically undermine—successfully—Canadian Athabasca tar sands and U.S. shale oil production (notably from N.D.'s Bakken Shale field).
Macro economics is exciting!