The Buffalo is a DHC5 with 2 x GE T64-10 engines. It is not a turbine DHC4 Caribou conversion. Different aircraft.
The PT6A-57 engine is already FAA certified in the DHC4 Caribou - but these are military aircraft and don't require civil certification.
My guess US$5 mill per airframe and Australia could retain the unique Caribou capability for another 20 years, with reliable turbines, refurbished and modern avionics etc.
Tell me again how much military aircraft you get for US$5 mill per airframe?