Sorry to necropost, but it needs to be done for posterity.
Back on topic. A Garrett turbine is about 20-30% more fuel efficient compared to a PT6. So a Turbo Commander with the -10 engine will burn about 380-400pph (that's about 58gal/hr) in total doing 290kts. That's about 100pph, or 14gals less than a PC12. That's having more horsepower and going faster. Overhaul also much cheaper than on the big PT6's. I would say it's a wash between overhauling 2x -10's vs 1x big PT6. So as you can see the myth that a single is always cheaper does not apply here.
Which begs the question - if there's no cost penalty for going faster, save fuel and save on purchase price - wouldn't you rather have a twin?