Sugar prices, driven up steadily this year by forecasts of a world shortage, have reached their highest level since late 1974. The reason behind the latest spurt was market reports that the Soviet Union, the world's largest producer, may have bought up to 3 million tons to make up for lost domestic production and imports from Cuba. Rain forced postponement of full harvesting of Soviet sugar and cold weather threatens to reduce yield severely, traders believe. Several other sugar producers have also had problems with their crops. Industry estimates say world demand in 1979-80 will be 91 million tons, 6 million tons higher than output.