What is the primary reason why a Pluto flyby mission would be cheaper than a Pluto orbiter?

Answer 1

To enter orbit, a spacecraft would need some sort of braking system to slow down and enter a stable orbit. It would either have to be some sort of "aerobraking" system or a propulsion system of some sort. Either would add complexity and cost to the spacecraft.

Answer 2

Im guessing fuel and sophisticated computers. If you fly by you can use its gravity to slingshot back. Otherwise you have to fly up to it get into its orbit. Slow down to the correct speed orbit and then come back. It would take a lot more fuel. If you just fly up to it you can use all the inertia to just fly by and slingshot back to earth.