I think we get about $5000 for a full unmachined casting set. (6 mag castings for the case/shifter/top, 1 aluminum bellhousing, 5 brass shift forks) Then you need several custom pieces including a custom ring and pinion. There's a reason new T44 boxes are more than $25K (Maybe more than $30K...)
As far as putting different internals in a modified case, check out the long thread on making your own gearbox.