Regen SW are arguing for a more 'balanced' approach to subsidy farming in the severn estuary...
"Bristol Channel Energy
A Balanced Technology Approach - Discussion Document"
They seem to want to compare investment returns after subsidy, rather than comparing underlying investment returns pre-subsidy. They don't discuss the economic benefit of putting a road across the barrage to link up Cardiff and Weston as part of a Severn Barrage scheme.
They do not analyse how the various schemes effect flooding risk further down the estuary. If it is to generate electrciity as the tide rises, a large lagoon may lead to higher high tides further up the estuary and associated costs incurred in terms of extra flood protection around up stream population centres.
It heavily quotes the "Severn Tidal Power Feasibility Study"
Conclusions and Summary Report
About Transport links this report says
Some commentators have suggested that a barrage across the Severn estuary should carry a new road or rail link. Such a link would be expensive as it would need to be elevated to provide adequate clearance for vessels to pass through locks. Such a development would not necessarily be less expensive than a separate construction structure. The feasibility study has commissioned work by Network Rail and the Highways Agency24 and has found no evidence that the existing road and rail infrastructure is inadequate or unable to meet anticipated traffic for at least the next 15-20 years.
It is possible that new transport links will be needed beyond 2025-30. With this in mind it would be feasible to accommodate suitable foundations either as part of the design of a barrage or subsequently by developing a design that adapted the existing structure for a future transport link. Issues associated with new transport links are not considered further in this report as further specific assessments would need to be undertaken at that time."
I don't think we know enough to rule out the barrage as the lowest cost option yet. It is always possible that the subsidy system acts to stop the most economically viable solution from being the one that ultimately succeeds.