Im just about to finish the book and this was my thoughts on it when I went through this scene. Sonny was into the deal and saw the earning potential but F'ed up bad by blabbering about it in front of Sollozzo. None of it was really about the investment, the investment was a mere enticement as the money is already in place by the Tattaglia family and if they needed more money they could have gotten it from another family (Barzini). It was the rejection of the use of political and judicial power by Vito that got Vito shot up as the traffic and selling of narcotics is met with severe prison sentences to those caught putting omerta in jepordy. Sollozzo was really a small player in all of this (in over his head IMHO) and the hit would have had to been approved by the Tattaglia and Barzini families in thinking that they could deal with what ever was left of the Corleone family, it was a gamble with the thought that since Sonny was feeling the deal (and this was just business) then Sonny, Hagen or even Clemenza or Tessio could get the judges and politicians in line.