Go under, not over
Richard Rogers recently said that of the 670 million spent building the Millennium Dome, only 46 million was actually spent on building it, so we clearly have a problem.
Technology has progressed since the previous bridge and the Norwegians have cut more than 700km of tunnels at 3.5 million to 10 million per kilometre. The tunnels for the Glendoe power station were also of this order. The government's reaction is to automatically reject it on NIH grounds (Not Invented Here), but a Forth tunnel could be cut for about 40 million.