Billiard with asteroids: apply trajectory intelligence and domino effect to move large asteroids through a sequence of small ones.
There are so many asteroids. To move a large asteroid, like Cruithne to Mars, you don't have to do it directly. Small asteroids could push larger asteroids slightly to collide with even larger ones, until a larger one is cought by a gravity well.
Rather than investing $ to lifting more fuel, it could be invested into getting more information -- surveying the surfaces, masses, spins, etc. of asteroids to make precision collisions.
Inspired by [asteroid to mars] idea.
Credits: Inyuki of HalfBakery.
Asteroid impactor mission - may have relevant people to talk to. Wikipedia link
This may be among the most realistic ways to move planets with small initial amounts of energy. Just have to map the trajectories of small bodies across the solar system.