Officials have warned that a detailed inspection of the two main suspension cables on the Forth Road Bridge is set to cost £750,000 more than expected.
Costs for checking the main cables have soared
The reasons for the price rise have been set out in an assessment to be considered next month by members of the Forth Estuary Transport Authority.
An increase in the cost of fitting acoustic monitoring equipment next year was largely to blame.
It has put up the bill from nearly £2.75m to £3.5m.
Costs have gone up because safety regulations are stricter than in the US, where the system is already used.
Authorities have said that likely delays in the installation process due to windy conditions also have to be taken into account.
The examination was ordered because of concern about the state of the cables and the need for a comprehensive analysis.
Contractors have discovered parts of the bridge have corroded
Checks already done on specially selected sections have revealed that a small number of the wires, which make up each part, were broken.
Experts have warned that corrosion will cause some others to fail.
Motorists avoided two weeks of disruption after work on the link finished ahead of schedule earlier in the year.
The first phase of the major maintenance programme was due to be completed on 19 June.
But the process, which included monitoring the condition of the main cables, had already been finished by 7 June.