I doubt Saudi will depeg the currency.
My take - they will hold on to the excess supply, though painful, till most of the US producers are driven off. Thereafter, OPEC just needs to cut 10% of their production (3Mio bbl/day) to counter the current excess of 2Mio Barrels per day, and prices will be back up - though not to $ 100's.
Meanwhile, the c. $100 B deficit will be funded by a combination of debt (expected $ 32B in 2016), utilising their $ 600B odd reserves, and subsidy reduction (last week local fuel prices were hiked by 50%). Subsidy reduction is taking place across GCC, and there's also talk of VAT/Sales tax.
Yesterday, banks were banned from trading SAR options, as the trades have been pressuring the forwards (on deltas).