Most 'modern' gas turbines with dual fuel capability can be switched automatically between either supply. The system is usually setup to transfer on loss of the primary fuel (e.g. loss of pressure). Also, most have the option to operate at a percentage of each fuel.
As always, Check OEM spec...
My latest copy is "Gas Turbine World 2000-01 Handbook". In that issue two plants were discribe near your need. One a MHI 701F (50 cycle) at $350/kw and GE Frame 7H at $500/kw. As noted actual cost will vary widely with number of turbines, BOP equipment, site requirement, environmental...
