If you enchanted Tower of Murmurs with Power Artifact, milling 8 cards would cost 6 mana.Quote from 3drinks »Also effect cost reducers. One of them the mana spent is directly tied to the # milled, the other is static.
If you enchanted Sands of Delirium with Power Artifact, milling 8 cards would cost 6 mana.
When activating Sands of Delerium, you choose the value of X, then determine the cost of activating it. In this case, you choose X=8, and determining the cost is 8 - 2 = 6.
It's not like Prossh, Marath, or Jeleva which care how much mana you actually spent. X-cost spells and abilities care what your choice for X was.