I reckon its more likely the costs are around planning and consenting than actual installation.
It is easy for the Council to say the hydro-excavation was needed for "health & safety" cause no-one can argue with H&S, and it is imposed by others, not the Council.
It would be a bit of a PR dissater if the Council fessed up and said:
"we've created such unwieldy complicated planning rules and District Plans that is cost us (the people that wrote the documents) moonbeams and 100's of hours of Consultants time, including charging ourselves for assessing our own application, just to work out if we could install some $400 buck welcome signs around the place".
If they came out and said that, there would probably be a revolt. Its more likely that is where the cost is - but we'll divert and distract with the old H&S chestnut.