IS installation IS less cost effective in Cl.I, Div 2 location than open raceway wiring that is allowed for items that may be available as IS devices. It MAY be more cost effective in Div. 1 areas that should be very limited in comparison to the Div 2 area in the same plant.
We have run cost analysis over and over on this and suggest to anyone, who is contemplating the installation of it, to do so. Key items are the entirely different components that need to be kept rigourously separated from other systems and even from each other if they are different IS systems. IS installationb and maintenance requires people with specialized skills that is not widely available so you will pay premium or you have underqualified people potentially screwing up the safety of your system.
Just be honest with yourself and think practical. The theory is great the reality is bleaker.