Drivers: How to invert values rather than copy?

See below, I am trying to make the cubes outside the plane be “mirrored” against the edges of the plane, so if the cube in the middle moves towards an edge, the “reflection” cube outside of that edge moves towards this.

How is this done?

Have got it to copy the x values and the y values to keep the reflected cubes on the same place, but have no idea how to not copy the other axis, but invert it.

Not sure if this is what you are after, I put the driver in delta transform (so you can move the outside cubes) and multiplied by -1 for the ones further away I multiplied bey -2 etc.
Mirror.blend (99.1 KB)