Not sure how much it cost to develop Sovaldi, but in general it costs $1-2Bn and ~10-12 years to develop a drug and only 1% of drugs in pipeline actually reach the market. So $11b seems a reasonable cost to assume.
That $1-2 billion figure pharmaceutical companies like to throw out is the risk adjusted cost, which includes all of the failed drugs and clinical trials for each successful product. The acquisition price for Pharmasset included Sovaldi and several other drugs.
