Correct, once you've announced that you're casting have and tapped your 1mana, you're good.
October 6, 2016 9:57 a.m.
The moment you announce you're casting Metalwork Colossus, it's too late for your opponents to remove one of your Aradara Express's. During the casting process, you determine the casting cost for your spell which includes any cost reductions.
TL;DR: You will only pay .