A council report on fixed links has estimated the cost of building five tunnels - including to Feltar - at around £500 million.
The Shetland Inter-Island Transport Connectivity report sets out the processes, timescales and resources required in delivering fixed links and ferries.
It covers the four tunnels usually discussed in the debate - to Unst, Yell, Whalsy and Bressay. And, for the first time, it also mentions a possible tunnel between Yell and Fetlar.
It says the total cost "could be in excess of £500m at 2022 prices".
The report also highlights the cost of ferry operations - estimated at £23m for the coming financial year - and the uncertainty as to whether it will be fully covered by the Scottish government.
In addition to the annual running costs, it highlights the capital investment needed for replacement ferries and infrastructure - estimated at around £120-150m.
The report asks councillors to authorise a project team to be set up to develop an "inter-island transport connectivity network" business case.
The cost, which includes surveys, reports and procurement of services such as project management, is estimated at up to £700,000.
The report will be discussed by the SIC's environment and transport committee on Tuesday.
It had originally been scheduled to be heard in private due to containing exempt information.
However, the report was reworked so that it could he heard in public.