Russia's current budget relies extremely heavily on oil revenue, such that the entire country's fortunes rise and fall with the price of oil. How was the Soviet budget financed, especially during the Cold War when it was trying to keep up with the US?
There was no 'main source' of revenue for the Soviet State, in the sense that modern day Saudi Arabia is dependent on oil or Switzerland on banking. The Soviet Economy was reasonably diversified (if inefficient and with a strong emphasis on heavy industry) and the State pulled it's revenues from every area of economic activity.
Until the 1980s, the Soviet Union was largely self-sufficient. Foreign trade was never a major source of revenue for the State, comprising only 4% of GDP in 1985 (in comparison, trade made up 16% of a much larger American GDP in the same year, according to World Bank figures). There was an abundance of fuel, steel, timber and mineral resources, though some minerals like bauxite - for aluminum production - had to be imported. Grain was always a major import, due to adverse farming conditions and inefficiency, but this varied depending on the harvests.
They more-or-less had everything a country needs to operate as an industrial economy within their borders and were able to maintain that for most of the twentieth century.
It sounds odd to say, but the Soviet Unions' wealth really was in it's people. In America, private citizens own, produce and trade goods and services between themselves, and are taxed to fund state and federal budgets. The Soviet economy operated in the same way, but with the State as a both taxman, middleman and secondary actor in the economic system. The State had an unparalleled ability to raise revenues and an effectively captive workforce. Their revenues came from the economic activity of their citizens within Soviet borders and was not dependent on any single resource. Industrial production comprised 46% of Soviet GDP in 1989, agriculture a further 19%, retail and distribution 18% - compare that to the Russian Federation today, where 52% of state revenues (and 16% of GDP) come from oil production alone.
It was a weird system in the end. Internally the economy was disastrously inefficient and failed at meeting the basic needs of the average citizen, but from the outside it was a pretty robust system that functioned a lot longer than than anyone might have predicted.
The country-data site is a handy source on the Soviet Union (and seems pretty accurate as far as I can tell).
This blog is a good source on the soviet economy. The guy seems obsessed with graphs and charts, if that's your thing.
There's an interesting theory that peak oil was largely responsible to the collapse of the Soviet economy - you can read about it here.
I'm not trying to answer the question but I was under the impression that the gulags were an integral part of keeping the Soviet economy afloat. For anyone more knowledgeable than myself, would this be an accurate assessment?