Staff Boost math

I get how Boost works in chaos mode games, at least I think I do i.e. you can boost the staff spells by the number you choose when forging a new staff, but how exactly does it work in Law mode?

Law Mode hover says: Influences the reduction in mana cost for spells matching staff type.
So the question is influences how exactly? Say I pick the Boost value to be 15, how much cheaper exactly are my staff spells now?

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