I remember the shortage of sheetrock in the late 90's. Prices rose a lot for that and they could'nt make the stuff fast enough for a while.
This is one of the factors in the unpredictability. Often a shortage in supply in one factor can screw up a whole job by causing unanticipated delays all down the line. Thus, while sheetrock prices may rise so that another 10 grand has to be added to the price the actual impact may be much greater if it causes an extra month to be added to the construction timetable.