i learned with my set (in progress) that players like bonuses more than penalties, even if they read the same. i would make it 2UU with As you cast ~, you may reveal an artifact and/or an instant card from your hand. for each card revealed this way, ~ costs less to cast.

That wording doesn't quite work - since the idea is that you can reveal something that is both and get both reductions - but your point of it being better as reduction than as cost is a good one.

While it isn't actually impossible, I'm not sure what an Artifact Instant would be like, so Mshnik's suggetion works in practice.

However, c.f. Wren's Run Vanquisher