That is on top of the $310,000 the council will probably spend building a cage around the wooden walkway and topping it with glass so it can be viewed by the public.
The council's economic development committee approved the retain and display option on Monday, and the full council is expected to endorse that decision when it meets on August 18.
The $200,000 was made up of archaeology costs, engaging a wood conservation expert, the cost of slow digging to uncover all the causeway without damaging it, geotechnical advice, additional design and management costs, and additional costs incurred with subcontractors having to vacate the site and return later, project manager Dave McKenzie said yesterday.
The combined cost of both the investigations and the retain and display option would be met from the contingency fund for the $21 million project, he said.
"We're talking about $510,000 all up, which is about 2.5% of the total cost of the project. It's a tough ask."
One way to ensure the causeway is permanently conserved is to spray it with polyethylene glycol. The cost of that could be as high as $500,000 over five years.
Historic Places Trust Otago-Southland manager Owen Graham said it was not yet known whether the PEG treatment would be required.
Two wood conservation experts, one a trust staff member and the other a private consultant, would inspect the causeway today.
The cost of bringing the experts to Dunedin would be shared by the trust and the council, he said.
• Polyethylene glycol
What is it? Common chemical manufactured as byproduct of oil industry.
What does it do? A loosening agent that causes reaction between water and liquids and gases which are not water.
What is it used for? Common ingredient in many everyday products, including plastics, glue, paint, cleaning products, shampoo, pill coatings, medicines and toothpaste.
