How do you read the "Relative Cost" of skills?

[Laraso]

So I recently discovered that pressing ! on the skills screen shows the "relative cost" of skills. It makes absolutely zero sense.

My 7.9 Armour (+1 apt) has a cost of 6.7
My 13.3 Invocations (+1 apt) has a cost of 16.0

They are both the same aptitude, so why does one have a "lower" cost, while the other one is "higher"?

[WingedEspeon]

I believe that that is the cost to raise the skill one level. Because your invocations skill is higher, it takes more exp to raise one level.

[Laraso]

Yes that's what I thought too, but then why is the "cost" of my armour skill lower than the actual level it's at, while the invocations skill's "cost" is higher than its actual level? Logic would state that since they both have the same aptitude, they would both follow the same rules. That explanation would make sense maybe if armour said it had a cost of 9.2 or something, so it would match the invocations, but it doesn't.

[WalrusKing]

Every single one of the relative costs is relative to raising a 0 aptitude skill from 0-1 from what I recall.

[savageorange]

Yes that's what I thought too, but then why is the "cost" of my armour skill lower than the actual level it's at, while the invocations skill's "cost" is higher than its actual level?

.. What?

Costs are normalized, that's why it's called relative cost. The norm being

raising a 0 aptitude skill from 0-1

Being higher or lower than the level of the skill is of no significance.

Cost in unnormalized, unscaled apt 0 skill points:

0->1 = 50
7->8 = 400
13->14 = 950

Skill points required to raise a 0 apt skill from level 0 to 1, for your character currently ~= 59.7.

6.7 * 59.7 = 400
16.0 * 59.7 = 955 skill points

Seems close enough to correct to me.

[ydeve]

Yes that's what I thought too, but then why is the "cost" of my armour skill lower than the actual level it's at, while the invocations skill's "cost" is higher than its actual level? Logic would state that since they both have the same aptitude, they would both follow the same rules. That explanation would make sense maybe if armour said it had a cost of 9.2 or something, so it would match the invocations, but it doesn't.

Because skill costs don't increase linearly per level. They are following the same rules. If both your Armour and Invocations were at the same level, you'd see the skill cost to the next level would be the same. But skill costs increase dramatically as the level increases, and as you've seen, going from 13->14 skill is about 2.5 times more expensive than from 8->9 skill.