According to this personal appeal from Wikipedia founder Jimmy Wales, the website is run on $10 million a year. He also says it is the fifth-most-viewed website in the world.
Could there be a point where Wikipedia could get too big to fail and would be granted a major influx of funds from some federal government? Has that point passed already? We could say that if Wikipedia failed, something else would take its place. But Wikipedia is already so implanted to our lexicon and view of the world… it seems like it would be hard to give it up.