@ Pablo.. you mention that you are in the roofing stage. A great percentage of construction costs is in the finishes stage i.e ceiling, floor tiles, bathroom fixtures septic tank e.t.c
Generally, foundation + ring beam/collumns + walling = roofing= +- 50 to 60 % of construction costs.
See the link below for a breakdown of costs at each stage.
http://picasaweb.google....3ooZjyTQ?feat=directlinkAs Iron Sharpens Iron, So one Man Sharpens Another.