IIRC, power increases damage by .23 per point while cunning increase damage by .2 per point.
for one overkill augment (with 18 power), you get 4.14 points of damage
for one cunning augment WITHOUT gearbox, (18 cunning), you get 3.6 points of damage
for one cunning augment WITH gearbox (18 cunning x 1.09 increase from gearbox= 19.62 cunning) which gives you 3.924 points of damage.
if you use only overkill augments, you get 14 x 18= 252 power which gets you 57.96 points of damage.
if you use only cunning augments, you get 14 x (18 x 1.09=19.62)= 274.68, which gets you 54.936 points of damage
No guarantees on the math though. I may not understand the values or the mechanics correctly.
However, cunning does increase your crit chance, which can factor in. The fewer points you have to spend on crit, the more you can put into power or cunning.