It doesn't matter what the common price was when the warrants were issued. What you fail to recognize is that the warrants were issued in conjunction with the senior preferred stock. So it wasn't just given away for essentially free.
"In exchange for Treasury’s financial support, the SPSPAs require Fannie Mae and Freddie Mac, among other things, to make quarterly dividend payments to Treasury, provide Treasury with a Liquidation Preference, and beginning in 2010 pay Treasury a periodic commitment fee that reflects the market value of the outstanding Treasury commitment, as well as Stock Warrants for the purchase of common stock representing 79.9% of the common stock of Fannie Mae and Freddie Mac, respectively, on a diluted basis."