If by some miracle that level of value is ultimately recovered over the next 10 years we could perhaps do worse that using the proceeds to pay off some of the national debt and invest in some major infrastructural projects which reduces our long term dependence on CO2 intensive transportation.  The costs you outline don't seem outlandish, although the government has a track record of mismanaging infrastructural projects to the extent that they come in at two or three times the original budget.  

The Chunnel experience is not encouraging.  Have tunneling technologies, techniques, and cost factors improved dramatically since?  No doubt prevailing ideologies would require some PPP type funding architecture which would require a huge risk premium to attract private investment.

notes from no w here