Take a look at
http://www.ap.urscorp.com/Projects/Details/?id=67&Service=Engineering&Sub=GeotechnicalTunnelling
Apparently, it cost $103M to dredge over 10Mt of 'hard' rock from the Geraldton Port. This number also includes navaids, breakwater construction and more. It took 10 months. If the limestone at Gero was Groote Mn, they would have made over $3.5Billion in 10 months at todays prices.
