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A ship is not something which you would normally go and buy immediately (of course this can be said about boats or second hand marine engines or ships) but most of the ships are custom built for the clients by shipyards across the world. Since a customer would do quite a bit of shopping around before they actually order a ship, the shipyards usually give an approximate estimate for the ship cost and timeframe of construction in order to bid for a particular order.
Herein lies the dilemma for if the shipyard grossly overestimates the cost, they risk losing the order to a competitor, but if they quote too low a price then they might end up paying from their own pocket instead of having a profit at the end of the day when the ship is constructed and delivered. Hence it is very important to correctly estimate the construction cost of a ship within a reasonable error limit.
Just to give you a rough idea about the scale of the costs, take a look at the graph given alongside and you will see how the shipbuilding costs have risen over a period of time of the past 7 decades as well as the percentage change in the ratio of men and material required for ship construction. You will be amused to note that the current costs have risen nearly 12 times since then which is understandable but another interesting aspect is that the cost of skilled personnel has grown faster than the cost of material for constructing a ship these days.