CHESHIRE, Conn. (TheStreet) -- Alexion Pharmaceuticals is buying Synageva Biopharma for $8.4 billion to broaden its offering of treatments for rare diseases, the companies announced Wednesday. The cash and stock deal values Synageva at $230 per share, based on Alexion's average closing price for the past nine days. That's a 140% premium to Synageva's closing price of $95.87 on Tuesday. Alexion is already one of the most successful marketers of so-called "ultra orphan drugs" -- treatments for rare, genetic diseases which carry gigantic price tags.