Air cargo is probably the majority of that 1.8 billion gallons. Then there are chartered aircraft and aircraft owned by corporations and other wealthy organizations. The helicopters operated by news organizations. The aircraft used to supply areas where ships and trucks cannot go, e.g., Antarctica. The vast majority of the aircraft I just described are operated by professional pilots.

A turboprop engine will cost millions of dollars in purchase cost and maintenance cost over its lifetime just like other jet engines will.