As a cost comparison, Hinkley Point C nuclear power station (also being built on the Severn Estuary) will cost £25bn, and deliver 3.2GW of power sold at £92.50 per megawatt hour (MWh) of electricity generated for the 35 years of the contract.[49] The Hafren scheme proposers state they would require £25 billion capital investment, and power costs would be £160 per MWh for the first 30 years, and £20 per MWh thereafter. Other schemes have been costed at between £150 and £350 per MWh.[2]
The other complication with tidal is that the output capacity varies with tides etc which also vary over the year and isn't constant. There seems to be some tax payer funded costs due to it's presence. Those are predictions.
Both are PFI funded via bank loans. I don't think Hafren exists any more ( 1/6 of shares own by a Tory MP's family business)
I've seen mention of yet another study.
Some one costed greening. Reported on C4. £7.5b 5 of those by the public and 2.5 by the gov which we also pay one way or another.