porting coldfusion app to rails [business case for:]

hi:

our company has a cms written in coldfusion, which is starting to show
its age, and we are contemplating rewriting it in rails - my boss has
asked me for some info that will enable him to determine if porting the
cms to rails will be good for the business.

there are many issues in such a project, but the main one that i am
unable to get a handle on (negligible rails experience) is the number
of man-hours. can anybody offer any insights in how best to calculate
the time involved in such a port.

thanks
mark