Theoddone
Flying in the UK, you would save up most of the HSI cost from the fuel price differential. Is it really £100k? £50k was what I've heard for a PT6.
Smaller engines seem better than a PT6. The Grob 140 uses an Allison (R/Royce) 450hp turbine, and being unpressurised the airframe maintenance costs are far lower. The Extra 500 looks good too.
The purchase prices are high though and I agree about utilisation. But... look at the huge operating costs of a piston twin, over many years, and a single turboprop gives you about 5x the reliability of a piston twin
