In the past 12 months, New York City mayor and potential real estate tax increaser Michael Bloomberg saw his personal fortune skyrocket some $8.5 billion. That's according to Forbes — which pegs his current wealth at $20 billion — which is basically a magazine that exists to estimate about this stuff. How'd the price kidnappers would demand for his release jump so much?

Forbes boosted his ranking because the company Mr. Bloomberg founded, Bloomberg L.P., bought back a 20% stake in the financial data and news firm held by Merrill Lynch this summer. The purchase price–$4.5 billion–implied a value for all of Bloomberg L.P. of $22.5 billion. Since Mayor Mike owns 88% of it (held in a blind trust), his personal net worth would be around $19.8 billion. Add in his homes, art, etc., and you get to $20 billion. Previously, Forbes guessed the company was worth more like $13 billion.

This is bad math, says the Wall Street Journal, for two reasons: 1) The price paid for that stake may not reflect the price Bloomberg L.P. would fetch if it were sold as a whole (i.e. Nobody out there would buy Bloomberg LP at that valuation); and 2) If Bloomberg were publicly traded, its stock likely would have taken a dive this week (you know, because with two Wall Street firms closing, that's two fewer companies buying Bloomberg terminals).

Also not factored into the equation: The gobs of cash Bloomberg is going to have to spend to convince all of New York to change term limits and let him be mayor for another four years.