Mohammed Idrees used to travel to London once or twice a year, but these days the Saudi civil servant is asking his wife and children to cut back on using the family car to save fuel and has installed a solar panel for the kitchen to reduce electricity costs. For decades, Saudi nationals such as Mr. Idrees enjoyed a cozy lifestyle in the desert kingdom as its rulers spent hundreds of billions of dollars of its oil revenue to subsidize essentials such as fuel, water and electricity. But a sharp drop in the price of oil, Saudi Arabia’s main revenue source, has forced the government to withdraw some benefits this year—raising the cost of living in the kingdom and hurting its middle class, a part of society long insulated from such problems. Saudi Arabia heads into next week’s meeting of major oil producers in a tight spot. With a slowing economy and shrinking foreign reserves, the kingdom is coming under pressure to take steps that support the price of oil, as it did this month with an accord it struck with Russia.