According to James Simons, "The average annual income of leading research mathematicians (those, say, with at least three articles in the Annals of Mathematics) is about 10,000,000 USD" ;]
Simons is worth several billions ($10 billion perhaps?) and the number of mathematicians with at least 3 articles in the Annals of Mathematics cannot be huge (1000 perhaps?). I am pretty sure that was what Simons was getting at. Seeing your smiley, I assume you knew that too!
I'm skeptical of that. There is one web site (besides the above comment) that has this quote. It links to the Wikipedia article about James Simons as its source. The Wikipedia article does not seem to mention the quote. Even if James Simons did say that, I'd like to see his source.
Edit: it just occurred to me that, since we are talking about "average" and not "median", this may as well be true, if a handful of mathematicians make a gazillion dollars a year. Very senior faculty members in the top research universities in the US are likely to make only $200-500k per year.