A proposal put forward for a tunnel as an alternative option for the planned M4 relief road would cost almost £10bn, it has been estimated.
The Welsh Government wants to build a £1.1bn six-lane motorway south of Newport to relieve congestion on the current M4 between Magor and Castleton.
But a public inquiry is looking at 22 alternatives to Welsh Government's preferred 15.23 mile (24km) route.
The tunnel proposal is 10 miles (16km) and has been suggested by a resident.
Of the 22 alternative options, nine alternatives have been proposed by residents while others have been mooted by other opponents to the M4 scheme.
The tunnel idea would see it start near junction 23A on the current M4 at Magor, travel under Llanwern steelworks and Newport docks, and return to ground level west of the Newport suburb of Duffryn.
But the Welsh Government said a tunnel would cost about £9.8bn which would make it one of the UK's most expensive transport projects.
It has claimed the plan has "significantly greater capital, operation and maintenance costs" than its preferred route, which it hopes to open in 2021, and the "cost of this scheme would greatly outweigh the benefits".
The preferred route has two planned junctions at Glan Llyn, near Llanwern steelworks, and at Newport docks.
But the tunnel proposal is without intermediate interchanges, which would lead to "reduced accessibility into Newport and reduction in road user benefits".
The Welsh Government has acknowledged the plan would have reduced environmental impact on the five sites of special scientific interest (SSSI) on the Gwent Levels than its preferred route - but would have "significant adverse impact" during construction.
The Welsh Government has argued the current M4 around Newport, opened in 1967, "does not meet modern motorway design standards and "is not fit for purpose".
Businesses insist the regular congestion around the Bynglas Tunnels on the current motorway is affecting the Welsh economy .
The alternatives to the planned route are being heard at a five-month public inquiry into the scheme.
Villagers in Llanfihangel Rogiet, near Caldicot, have submitted an alternative route as they say their Monmouthshire hamlet would be "blighted" if the Welsh Government proposal gets the go-ahead.
Welsh transport professor Stuart Cole will give evidence to the inquiry on Wednesday proposing a cheaper £776m "blue route" on behalf of Cycling UK.
That proposal is for the current southern distributor road around Newport to be upgraded to motorway standard and also upgrade the new steelworks access road between Spytty and Magor, linking to the M4's current junction 23A.
The owners of Newport docks and Magor motorway services will also give evidence at the inquiry, promoting their suggested alternative routes - ranging between £1bn and £1.5bn - as they claim the Welsh Government proposal will detrimentally affect their business.