So is skill involved at all (other than prerequisites for perks) or is the magnitude of all enchants based on perks and soul gem used?

For example:

Starting skill of 15 + Grand Soul Gem + no additional perks = +20 Magicka

vs.

Skill of 100 + Grand Soul Gem + no additional perks = ?

Yes.

For instance.

Skill of 80, grand soul gem, all 5 enchanter perks, the insightful enchanter perk = 23% destruction reduction in cost.
Skill of 100........... = 25% destruction reduction in cost.
Skill of 100........... + 25% enchanting potion(which temporarily raises skill to 125)= 27%(I think) reduction.

Skill does matter, I'm sure there is a calculator that is exact.

Sounds like skill is extremely minimal. I wonder what the values would be if you just took one Insightful perk, then went up the middle. Not a huge bonus, but only four perks spent.

When I do Enchanting I go straight up the middle and don't take perks from the right or left sides of the tree. For an armored character, that's all the perks I will ever need. I do take all the ranks in the first perk, though, if I'm making Enchanting a primary skill.