Summary: Upon further testing with the help of other players trying different staves, enchants and reforges we have found that cast speed reforges are the cause of abnormally high MP costs for INT magics NOT mana reduction enchants like I initially thought. My current theory is that cast speed reforges interfere with the mana reduction provided by high int (Mana Consumption and Casting times of Thunder, Fireball, Ice Spear, and Hail Storm are decreased by 2% per 100 Int) it seems the cast speed reforge breaks the formula in an uninteded way.
Difficulty: Intermediate, easy to trigger but fairly detrimental to the player. Triggered by simply having cast speed reforge on a staff (and probably wand?).
Occurrence: Always when casting INT magics with a staff that has any amount of cast speed reforge, cost seems to get higher with higher cast speed reforge level? probably makes hailstorm cost more too haven't tested yet.
Details: After testing with different staves with and without mana reduction enchants, with and without cast speed it seems only staves with cast speed reforge exhibit abnormal MP costs, strange ES doesn't appear to cause the issue.