You begin with the fact that equipment that is arcing, sparking or heat producing (ASH) will generally need to be in sealed, explosion-proof enclosures in either Division; so essentially that is a fixed cost.
After that, things get messy - especially for Group B. It is very difficult to summarize here but, excluding intrinsically safe systems, for most installations everything including conduit fittings will need to be or in explosion-proof enclosures in Division 1 and not everything needs to be in Division 2.
Motors will be a particular problem since very few are listed for Division 1, Group B use although Division 2 applications are fairly easy to achieve.
Typically the cost of a Division 1 installation will be 35 to 75% higher than one for Division 2 with similar operating functions but it is impossible to be more specific without knowing the actual scope of the project.
I would look very carefully at the classified envelope. Review both NFPA 497 and API RP 500. You will probably find the locations that actually need to be classified Division 1 are few and relatively small.
It is rather typical to grossly over-classify ("just to be safe") and then when the price tag begins to show to attempt to avoid the consequences by “cheating” in the Division 1 location. This often leads to an installation that wouldn't be safe in Division 2 either.
Last edited by rbalex; 06-09-06 at 12:49 PM.
Robert B. Alexander, P.E.
Answers based on 2014 NEC unless otherwise noted.