Just to clarify
A spell can only last 1 turn
A permanent can last N turns (N is variable)
A status can last M turns (M is variable)
Since Sandstorm affects all creatures, not just all creatures when it was cast, it needs to be a permanent not a spell that inflicts a status.
Furthermore
The unupped causes ~20% per turn for 3 turns, The upgraded causes ~17.5% per turn for 6 turns.
Additionally this does not take up a unique slot (even if it were a permanent) therefore it does not get the -3 cost reduction that shields and weapons seem to have. However this is counteracted by the smaller percentage and the shorter duration.
I think a casting cost of
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would be balanced.