What kind of cost is the effect on Trinisphere?

At a tournament this past weekend my opponent controlled a Trinisphere. I drew a Temporal Mastery as my first card for the turn, and the judge told me that it costs three to cast.

Five minutes later against the same opponent, I had a similar question about Gigadrowse. The judge said that I need to figure out the total cost, including whatever I would pay for Replicate, and if its still less than three, then I pay three.

I want to learn how to calculate the cost without calling a judge, but I can't figure out what kind of cost Trinisphere's effect is. It doesn't seem like an alternative cost, additional cost, or a cost increase. What kind of cost is it?

• total cost = ( mana cost or alt cost ) + additional costs and cost increases - cost reduction. Trinisphere modifies this total cost. – ikegami Jan 30 '18 at 16:00

Trinisphere's ability is an effect that directly changes the cost and takes place after all cost reductions and increases have been taken into account.

This is covered in the rulings on it's gatherer page:

Trinisphere’s ability affects the total cost of the spell. It is applied after any other cost increasers or cost reducers are applied: First apply any cost increases. Next apply any cost reducers. Finally look at the amount of mana you have to pay. If it’s less than three mana, you’ll pay three mana.

Even with a cost reducer on the battlefield, spells can’t cost less than three mana to cast.

If a spell costs at least three mana due to additional costs, such as kicker costs, that’s fine.

So to simply determine the costs for spell that you cast normally while a Trinisphere is on the field you can:

1. Start with the mana cost or alternate cost of the spell. (Temporal Mastery's Miracle is an alternate cost.)
3. Add any cost increases (effects like Chill).
4. Subtract any cost reductions (effects like Baral, Chief of Compliance).
5. Check to see what the total cost of you spell is, if it is 0, 1 or 2 CMC you will pay 3 instead. If it is 3 or more CMC you will pay that cost.

Note that this order is the same order outlined in the comprehensive rules (emphasis mine):

601.2f: The player determines the total cost of the spell. Usually this is just the mana cost. Some spells have additional or alternative costs. Some effects may increase or reduce the cost to pay, or may provide other alternative costs. Costs may include paying mana, tapping permanents, sacrificing permanents, discarding cards, and so on. The total cost is the mana cost or alternative cost (as determined in rule 601.2b), plus all additional costs and cost increases, and minus all cost reductions. If multiple cost reductions apply, the player may apply them in any order. If the mana component of the total cost is reduced to nothing by cost reduction effects, it is considered to be {0}. It can’t be reduced to less than {0}. Once the total cost is determined, any effects that directly affect the total cost are applied. Then the resulting total cost becomes “locked in.” If effects would change the total cost after this time, they have no effect.

• I'm pretty sure that effect is not actually a replacement effect. Rule 614.1 defines what counts as a replacement effect, and by those standards Trinishpere's ability doesn't qualify. – murgatroid99 Jan 30 '18 at 17:48
• @murgatroid99 You are right, I think there isn't really a game term for it other than maybe "effects that directly affect the total cost". Will change wording so as not to confuse. – Malco Jan 30 '18 at 17:52
• Most unusual static abilities are probably in the category of continuous effects that affect the game rules, which is mentioned in rule 613.10. – murgatroid99 Jan 30 '18 at 17:58
• Note also that Trinisphere does not affect alternate payment methods (such as Delve, Convoke, or Improvise). Treasure Cruise can be cast for U and 7 cards from your graveyard under Trinisphere. – eyeballfrog Jan 30 '18 at 23:28

Trinisphere's ability affects the total cost of a spell, and it needs to be applied after any cost increasers or reducers are applied. To calculate the cost of a spell for Trinisphere, you need to regard the casting cost of that spell at the time it's cast, not its converted mana cost. That's why cards with alternative mana costs such as Temporal Mastery still require 3 mana to be cast. So, to figure out what you need to pay for the spell, just think that if you'd cast that spell for anything less than 3 mana, it'll cost 3 mana. For example, if you chose to replicate Gigadrowse only once, you'd need to pay {U} for the spell and {U} for the replicate cost. That would make the casting cost of Gigadrowse {U}{U}, which is less than 3. Trinisphere's ability would then force you to pay {1}{U}{U}. The same effect would apply, for instance, if you have a cost reducer in play (such as Baral, Chief of Compliance), and played a Seething Song. Seething Song's CMC is 3, and Baral would reduce its cost by {1}. Since Trinisphere's ability is applied after increases/reductions in cost, Seething Song would still cost {2}{R} to cast.