You might want to check on the following two websites to get some idea as to what it might cost. Remember, if built in the US, costs would be significantly higher.
http://www.apolloduck.com/
http://www.midcoastboat.com/
Check under commercial > cargo, and then look for new builds.
Cost
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Guest
Re: Cost
The first couple Coast Guard 225 ft Buoy Tenders built at Marinette Marine in the mid to late 90's were around 45 Million. I think the cost went down some after the first five or so to probably around 38 million. I am sure that a freighter would be less as they would not need the buoy handling eqp, state of the art buoy positioning electronics and crews quarters that the tenders have. Very nice ships and a great job by the yard on design and construction.