The smartlink consists of items in multiple locations; 3rd edition had a great breakdown, which is essentially this: the palm induction pad (0.1 Essence), the eye display (0.1 essence, which can be piped through a display link if you have one), a limited simsense rig (0.1 essence, so the system knows where the gun is and is pointed in relation to your eye), and the smartgun processor (0.2 essence). Added together, this costs 0.5 essence. When part of a cyberlimb, the total is reduced to 0.25 essence - as with eye/earware, sometimes the implant alone is going to cost a lot of essence, while as part of a replacement it's not going to have much impact. In this case, the palm induction pad is part of the hand (-0.1 essence), the limited simsense rig is partially reduced (-0.05 essence) because it's being piggybacked on the 'where is my hand in relation to the rest of me' software required as part of the limb, and you also get a partial reduction in the smartgun processor (-0.1 essence), because some of that chipset is going to be part of the palm feed.
However, the 'space' required is pretty negligible; 0.5 ECU (equipment capacity units) for the thing, and if you want to get a second induction pad in your other hand (even though you can't by the rules use both at the same time) an induction pad takes up 0.25 ECU.
For reference:
Replacement | Obvious | Synthetic |
Hand/Foot | 2 | 1 |
Full Arm | 10 | 5 |
Full Leg | 20 | 10 |
Upper Arm | 6 | 3 |
Lower Arm | 4 | 2 |
Upper Leg | 12 | 6 |
Lower Leg | 8 | 4 |
Skull | 2 | 1 |
Torso | 7 | 3 |