So a typical fuel burn of 34gall per hour is double? Typical fuel burn for an IO540 PA46 is 24 gallons per hour (I know from first hand experience). Plus an initial climb rate of 3000ft per minute in the Jetprop rather than 1300ft / min in the piston variant.
Using December 2011 Birmingham UK fuel prices that works out at approximatly £0.91 / L for Jet A1 and £1.94 for Avgas.
In my book that equates (approx) to £3.44 per gall (US) and £7.34. therefore the Jetprop cost approximatly £116 / hour in fuel as against £176 per hour for the Avgas. So a Jetprop fuel costs approximatly 35% less than the same airframe powered by an Avgas burning engine. If we factor in the expected TBO figures then your way ahead.