So there's something I need to check. Does delve get around Trinisphere? The reasoning I heard is that because Delve has "Each card you exile from your graveyard while casting this spell pays for 1," when Trinisphere sees the delve card in question... we'll go with Treasure Cruise for this hypothetical... Trinisphere "sees" that 8 mana was paid even though 7 cards paid for the 7.
Does this work or did the player in this situation pay 2U + 7 cards that were exiled before the Trinisphere checked?
Paying a spell's costs using delve isn't actually impacted by Trinisphere. Under old rules, delve reduced the cost of the spell for each card you exiled from your graveyard, so you'd exile some number of cards, then Trinisphere's effect would apply and force you to pay three mana for the spell, regardless of how many cards you exiled. This is no longer the case - exiling cards due to the delve ability actually pays for 1 of the spell's mana cost. Therefore, since Trinisphere's effect applies before costs are paid, it doesn't actually do anything anymore.
1) You announce that you're casting Treasure Cruise.
2) The cost is calculated by applying any cost reducers or increasers - this is where the old delve rules applied and you'd exile cards to reduce Treasure Cruise's cost.
3) Trinisphere's effect is applied - if the cost of the spell is less than 3 mana, it becomes 3 mana (specifically, 2U).
4) You pay Treasure Cruise's costs, including exiling any cards to the delve ability.
Since Trinisphere's effect is applied before costs are paid, you can end up paying less than three actual mana for Treasure Cruise, but as far as the game is concerned, you paid the full 7U cost. The game doesn't go back to apply Trinisphere's effect, so it doesn't actually affect casting the spell.
Does this work or did the player in this situation pay 2U + 7 cards that were exiled before the Trinisphere checked?
Card tags added for Trinisphere. -MadMage
candidus inperti; si nil, his utere mecum.
~~~~~
In the case of Treasure Cruise, it goes like this:
1) You announce that you're casting Treasure Cruise.
2) The cost is calculated by applying any cost reducers or increasers - this is where the old delve rules applied and you'd exile cards to reduce Treasure Cruise's cost.
3) Trinisphere's effect is applied - if the cost of the spell is less than 3 mana, it becomes 3 mana (specifically, 2U).
4) You pay Treasure Cruise's costs, including exiling any cards to the delve ability.
Since Trinisphere's effect is applied before costs are paid, you can end up paying less than three actual mana for Treasure Cruise, but as far as the game is concerned, you paid the full 7U cost. The game doesn't go back to apply Trinisphere's effect, so it doesn't actually affect casting the spell.
There's a MTG Judge blog entry about this interaction.