Mighty/tremendous blow energy cost is applied even when aborting an attack
If you use a mighty/tremendous blow against a nonhostile monster and get the 'Really backstab the..." query, and answer No, the energy cost of 2500/4000 is still used. Considering that this is (IMO) more of an interface thing rather than the PC seriously contemplating it while actually preparing for the blow, the energy cost should not apply.