Per line of Mod Installation tree (red, Blue, Yellow, Green) whichever Skill you are in (Weaponsmith, Armoursmith, Engineering)
They cost 35 Perk points each line, And there are four. Which would mean 140 Perk Points spent just on being able to install all kinds of mods onto items.
I suggest combining them all into one set of Mod-Installation perk, and not make them limited by colour line. Drop the price from 140PP to 60-70PP Total per tree