goldeneaglepilot - having flown in both a 421C and the Jetprop and having a little bit of knowledge of both (more of the latter) I would not be suprised if the Jetprop turned out to be cheaper in all respects, except the original capital cost which is about £100k-200k for a 421C versus about $1M for a Jetprop (both obviously used specimens). A brand new JP is about $1.5M. And the 421C will carry more stuff.
I also agree that a Jetprop is the only logical way forward from an IO540 engined plane - which is why I am sticking with what I've got
The TB20 fits my mission profile.
But this comparison is at the very top end of piston GA, and works in favour of the JP due to the relatively huge avgas bills in the 421C.
Even a TBM700 costs less direct operating cost per mile than my TB20, but the purchase cost is $1.3M versus $200k for comparable specimens, or $3.2M versus $300k at new prices.
With such massive purchase price differences one cannot meaningfully talk about the operating cost being similar or whatever, because a million bucks buys you more avgas than most could burn in the rest of their life.