Just trying to find some site / tool that allows me to figure how much a set of effects at given skill proficiency would cost.... Any usedul stuff (I know the formula is Cost = BaseCost * (1.4 - 0.012 * Skill)

Graf here should work give you some tips
http://www.uesp.net/wiki/File:SkillVsManaEfficiency.jpg
As you see half of the gain happens after skill level 80.

http://www.uesp.net/wiki/User:Ong_elvin
Is an excel application for calculating spell and enchantment cost
Works pretty well as I can see.

Many thanx, Zaria. The xls stuff is kind of what I was expecting!