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.