According to [1] the cost of such a large wind turbine is about $1 MM/ MW. Assuming a capacity factor of 40% [2], a 12 MW turbine would produce about 115 MWh per day, or 42 GWh per year, or 840 GWh over a lifespan of 20 years. That's about 1.4 cents per kWh. But that's excluding other costs (installation, operation, transmission). It appears to me this is the dominant cost, so overall I think it would come to less than 3 cents per kWh.
Slightly smaller ones are about $2800/kW including finance, installation, substation, and (sometimes) cabling to shore. The turbine is 1/3rd to 1/2. Generally bigger ones are cheaper, but deeper water has costs too.
