New ATR-42, $20 million +- a few mil. New cockpit to cat 2 standard, possibly even cat 3, $500k to $1m. T700 core and engine is still being refined as its the basis for the Blackhawk/Seahawk helicopter. So it would not take a lot of imagination to get GE to refine the CT-7. Overall you could probably refit and zero time a SAAB 340 to a modern standard for under $10m. That is a full gut and refit with digital, avionics, led lighting, extended low weight composites, passive and active noise control, new engine and props etc etc.... Problem is right now how long will fossil fuel and its technologies power the aircraft. Even $10 mil needs to be spread over at least the next 20 years to be viable.