This is my system:
A character's base Constitution is his Cyber Form Score. He can have a certain number of augmentations that based on that score. It doesn't go up as points are spent on Constitution due to leveling up or by other modifiers. The character can take feats (Cyber Affinity, Greater Cyber Affinity) that will increase the Cyber Form Score, though Cyber Affinity must be taken at first level. Also there is an Experience cost in implanting Augmentations. This represents the time and energy needed to learn how to use the augmentation. Another feat (Chrome Soul) effects the base XP cost to lower it, which can only be taken at 1st level. The player can choose not to spend the experience but he takes a -4 penalty on all actions that makes use of the augmentation.
Also going under the knife also carries risk. as well, though I need to work on it more to make sense beyond myself.
Every piece of augmentation has a Purchase DC, but not all augmentations have a Cyber Form cost or cost experience to use. Also the character can choose to go beyond is Cyber Form Score, but takes temporary Constitution damage per day as long as the overloading is in place. The character can take expensive drugs to combat it but that works against his immune system which give a penalty of a -4 to all Fortitude saves.
As I stated I have 140+ different augmentations (cybernetics, biotech, and nanotech) that I and an friend of mine has written up. I also set it up a Cultural Technology Level system to separate different advances in technology in augmentations.
Here are three examples and tell me what you think.
Cybernetics
Cyberarm
CTL10
Surgery: Serious
Purchase DC 18
CFC: 2
XP Cost: 500
This is the standard replacement limb cyberarm which has the base score of your Strength and Dexterity, though at additional cost this can be increased. For every +1 to the base Purchase DC you can increase either you Strength or Dexterity by one point, max being a +4 Strength and Dexterity enhancement bonus, also each point increases the CFC by 0.5. These bonuses only apply to that arm. You can do similar upgrades to other limbs as well. A cyberarm has 4 spaces available for various options. For no additional CFC cost, however, you can increase your Strength or Dexterity by using up your option spaces. 1 Option Space equals 1 point of stat increase. Cyberarms have a base structural hit points of 20 (destroyed at –10) and a punch from a cyberarm does killing damage as its base instead of subdual damage. For a cyberarm constructed in a CTL 12+ culture the cyberarm has 6 option spaces available to it.
Biotech
Advanced Respiratory System
CTL11
Surgery: Serious
Purchase DC: 18
CTC: 1.5
XP Cost: 100
A new pair of lungs are grown and enhanced with extra alveoli, replacing the old lungs. The user can hold his breath twice as long and can effectively function at higher altitudes. The character receives a +2 enhancement bonus to his Constitution. This enhancement cannot be combined with other circulatory system enhancements.
Nanotech
Strength Augmentation
CTL12
Surgery: Light
Purchase DC: 22 per level
CTC: 1
XP Cost: 50 Per level
This system rebuilds the host’s muscles, tendons, and ligaments, increasing overall strength and reinforcing the body to allow it to take advantage of the new strength. The result is an enhancement bonus to Strength equal to twice the implant’s level. The power points needed to power this system equals to the implant’s level.
(max rating: 5)
A special note that the nanotech uses a core system that is intergrated to the body's natural energy which powers the various systems. If the core system doesn't have the power the various systems will begin to shut down.