In SR4 it is possible to buy cyberlimbs, and customize them with upgrades that cost a certain amount of capacity (noted [X] in relevant tables) rather than essence. This is different from a capacity available for installing mods (noted X, without square brackets.)
A cyberskull seems appropriate to house a pair of cybereyes, but I can't find any capacity costs for them. Cyberskull capacity and cybereye capacity seem unrelated since the skull has a capacity of 4, and a rating 4 cybereye has a capacity of 16, so I don't think it may be used as a capacity cost.
Augmentation p.38 talks about single cybereyes, telling:
Single cybereyes may also be installed in other parts of the body, independent of the character’s usual (cyber)eyes. These additional cybereyes can be implanted almost anywhere on a character’s body.
- Do cybereyes always cost essence, no matter where they are installed?
- If not, what is the capacity cost for cybereye(s)?