Historical Rules · 18 June 2021
Rule 202.3d
Official source text from the 18 June 2021 Rules release
The mana value of a split card not on the stack or of a fused split spell on the stack is determined from the combined mana costs of its halves. Otherwise, while a split card is on the stack, the mana value of the spell is determined by the mana cost of the half that was chosen to be cast. See rule 708, “Split Cards.”