If harbours were included in the structures, then that would drive up the costs. Another factor is the actual number of lighthouses that would receive a heritage designation. Depending on the standards, costs could escalate.
One example that is often cited is that of heritage train stations. If memory serves me well, 60% of train stations have been designated as heritage structures. If we go with a proportion of 60%, then we're closer to the $65 million estimate.