Affinity keyword rules question

[Mr. Froggy]

Lets say I cast Thoughtcast, while having 4 random artifacts in play, and my opponent has a Thorn of Amethyst on the Battlefield. How much does my Thoughtcast cost with Affinity? Will it cost just one blue, or does Thorn get added after? I know, probably a dumb question, but I'm curious.

[cdr]

Cost + cost increasers - cost reducers + minimum cost = total cost.

It doesn't matter which order you apply cost increasers and reducers in, but minimum cost is always last.

4U + 1 - 4 = 1U.

If you had 5+ artifacts it would cost U.

[Mr. Froggy]

It's a lot simpler than I imagined, lol. Thanks!

[Julian23]

Related bonus info: Note that the total cost is "locked in" before you proceed to actually paying it. Therefore, when casting Thoughtcast with 4 artifacts, one of them being Chromatic Sphere in play, you can use the Sphere to pay the blue mana required for Thoughtcast while having it still count for Affinity.