No idea, I've only ever seen white or pink diamonds, but believe blue ones are very rare. 1.25-1.75ct prices vary, also depends on inclusions and also the metal of the shank and band.
From what I've seen on websites, for a single solitaire, set on a platinum band, you are probably looking at a minimum of �5K.
Maybe you should contact a professional diamond dealer.
I know quite a bit about precious stones, and the blue diamond, being somewhat rare, but getting more popular, depends on its cut, as to the clarity and price.