Not sure how you're computing benefits of the tunnels...
Those tunnels cut travel time to the capital from 1h14 to 16 min. Moreover, you don't have to travel by car+ferry+car, you simply drive.
I reckon the increased accessibility makes it worth them in one year.
If you only want to count in monetary terms, consider that a hospital in the capital is suddenly reachable in 16 min instead of over 1hr. So they don't need helicopters for emergencies; they don't need minor, unsustainable hospitals on the other island just to have healthcare accessible, etc. Same for other things: supplying the 2nd islands can be done by lorry from the first, instead of shipping. Et cetera, et cetera, et cetera.
Those tunnels cut travel time to the capital from 1h14 to 16 min. Moreover, you don't have to travel by car+ferry+car, you simply drive.
I reckon the increased accessibility makes it worth them in one year.
If you only want to count in monetary terms, consider that a hospital in the capital is suddenly reachable in 16 min instead of over 1hr. So they don't need helicopters for emergencies; they don't need minor, unsustainable hospitals on the other island just to have healthcare accessible, etc. Same for other things: supplying the 2nd islands can be done by lorry from the first, instead of shipping. Et cetera, et cetera, et cetera.