There are all sorts of variables in the ways that costs are calculated for energy sources, but by most estimates, tidal power is the single most expensive way to go.
UK energy costs for different generation technologies in pounds per megawatt hour (2010)
Technology Cost range (£/MWh)
New nuclear 80–105
Onshore wind 80–110
Biomass 60–120
Natural gas turbines with CO2 capture 60–130
Coal with CO2 capture 100–155
Solar farms 125–180
Offshore wind 150–210
Natural gas turbine, no CO2 capture 55–110
Tidal power 155–390
Source: Parsons Brinckerhoff