A groyne trial at St Clair Beach in Dunedin could be worth more than $1million if approved.
Or, if all costs come in at the cheap end of the range, it might have a final price tag as low as $470,000.
Costs would include obtaining consent, constructing the groyne, monitoring its effectiveness and annual maintenance.
The Dunedin City Council faces a decision next week about whether to seek a resource consent to reinstate a groyne at the beach.
A trial groyne has a better chance of securing approval than a permanent structure, Dunedin councillors have been told.
It has been suggested a trial might run for five years, allowing time to evaluate whether a reinstated structure is effective in retaining sand.
The groyne might be pulled out if the trial is not successful or, if there are signs of success, a longer term might then be applied for.
Legal firm Anderson Lloyd said the council could choose to seek a longer-term consent, up to a maximum of 35 years, or a short term, "say for five years".
The consenting process, from the date of the application being lodged to a decision being made, could take six months.
City council staff estimated the cost of consent and construction could be about $500,000, or fall within a range of $345,000 to $720,000.
This consists of producing an assessment of environmental effects and technical assessments to support a resource consent ($100,000 to $150,000), legal costs ($50,000 to $200,000), consent-processing costs ($15,000 to $50,000) and constructing a single line of piles planked together ($180,000 to $320,000).
Consenting costs had previously been estimated at between $15,000 and $250,000.
On top of that, up to $50,000 might be spent annually on maintenance and between $25,000 and $60,000 annually on monitoring.
An attempt by the council to establish it did not need to get a resource consent, and should instead be able to get permission through a cheaper and less onerous process, failed last month.
The move was made despite receipt of legal advice success through this avenue was unlikely.
Dunedin Mayor Jules Radich, an advocate for groynes, said last month the manoeuvre was worth a try, "to see if you can do it the easy way first".
Knocked back on this bid by the Otago Regional Council, the city council now has to decide if it should pursue a resource consent.
Anderson Lloyd advised in November last year a trial "may be more realistic to consent" than a permanent structure.
It was likely an application for resource consent would require specialist advice on coastal processes and engineering, landscape and visual effects, marine ecology, and planning.
It was "reasonably likely" the application would need to be publicly notified, "because the environment is a public space, the proposal has the potential to affect the public, and there is a high level of public interest".
