RIYADH—The drastic slide in global crude prices is expected to force Saudi Arabia, the world’s leading oil exporter, to slash spending and cut back on the billions of dollars it spends on generous benefits for its citizens in next year’s budget.

The oil-rich kingdom spent hundreds of billions of dollars at home in the past decade to bolster its economy and dole out subsidies that provide cheap energy and food for its 30 million people, as it enjoyed years of high crude prices. But the price of oil has fallen by more than half...