How to forecast critical project cost?

If you are considering Pertmaster for this purpose keep in mind that Pertmaster uses CPM logic only to determine probabilities for a Job to be completed at various total job durations using Monte Carlo. For cost statistics Pertmaster uses Monte Carlo without tying costs to activities duration, for Pertmaster Cost distribution, CPM logic does not matter.

If by Critical Project Cost you refer to the optimum cost a Project can be scheduled it is a Time-Cost issue. This is an issue of time-cost-tradeoff Pertmaster does not handle. Monte Carlo as well as other Operations Research techniques can be used for this purpose. For a start you might follow the links:

http://books.google.com/books?id=sDRB7STAHXcC&pg=PA238&lpg=PA238&dq=critical+project+cost&source=bl&ots=bAWGgGJ-0a&sig=AhWPDA53TQVIV_dSVpfqCtDSr2U&hl=en&ei=j6MOSvy0KMnJtgeBvLSRCA&sa=X&oi=book_result&ct=result&resnum=2#PPA238,M1

http://www.scienceofbetter.org/what/index.htm

Into the practical, not into the theoretical, just understand the basics and search for a software application that solves the issue. I don’t believe Primavera has an application for this purpose but if a user of Pertmaster maybe you can contact them.