How much would a 11 carat blue diamond cost?

First, you'd have to find such a diamond. This size and colour are uncommon, blue being one of the rarest diamond colours.

Recently (21Oct2010), the Bulgari Blue Diamond, "...a triangular 10.95 carat Fancy Vivid blue diamond, (the largest such blue diamond of this cut ever offered at auction)," sold for $15.7 million or $1.4 million per carat. (From The Telegraph, London.)