I don't think this is that bad, this would be almost the same as casting 100 Buried Treasure, but I don't think this needs to be legendary.
Maybe rare, but takes 280 Turns to cast?
100 Buried Treasure will cost 300 turns, but with one rare cast to get 2.5 mill, that would be like a easier version to Buried Treasure.
So if you don't have 100 Buried Treasure one cast from this can get em easier?
I am trying to not change the idea as much as possible but instead to just make it so its balanced.
If saves you from many tiring clicks and it saves you 20 turns. At the cost of using up a lot of turns in one go.