If I use Temporal Portal on a spell that has reduced cost, will the repeat from the Temporal Portal cost the reduced cost or base cost?

When you use Temporal Portal to grant Repeat to a spell, the cost of that Repeat is determined by the card's printed cost, and cost reductions apply to the total cost of the spell (base cost + Repeat cost) rather than being applied twice.

According to the official rulings on this interaction:

1. Cost Calculation: When you play a spell with Repeat (or granted Repeat via Temporal Portal), you calculate the total cost as: (Base Cost) + (Repeat Cost) - (Total Cost Reductions).
2. Printed Cost: Anything that references a card's "cost" looks at its printed (base) cost. Cost reductions are passive abilities that apply during the cost determination step, but they do not change the card's printed cost for the purpose of calculating the Repeat cost.
3. Single Reduction: The cost reduction applies only once to the combined total of the base cost and the Repeat cost.

Example Sequence:

• You cast a spell with a base cost of 3 and grant it Repeat (which also costs 3).
• If you have a cost reduction of 2, the calculation is: 3 (Base) + 3 (Repeat) - 2 (Reduction) = 4 total energy.
• Upon resolution, the spell's instructions are executed twice.

In summary, the Repeat cost is not "reduced" separately; rather, the total energy you pay for the spell (including the Repeat cost) is reduced by the total amount of your cost-reduction effects.