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Wizard's staff reduces spell costs by 1 mana; wizard's cloak reduces spell costs by 1 mana. In total, spells cost 6 mana less. Split initially costs 12 mana. And, as a bonus, there's a crown – I don't remember the exact name – that gives: creatures in the hero's army spend 33% less mana (hat). Nightmares cast Fear 2 times.
Would you really refuse a cool artifact like the "Elven Bow" or "Horned Bow" or "Thunder Hammer," etc., in exchange for something from this list for a simple item that can be bought in almost every guild – a wizard's staff?
Even if we don't consider such cool artifacts, but take something simpler (guardian's halberd or angel's blade or staffs of various schools of magic, etc.). And if we talk about headgear, then it's better to have the Great Wizard's Crown. Only it would make me take off, for example, the Helm of Power, the Mask of Terror, the Crown of Spell Power, etc.