How much it cost Union to Port to iOS?

My understanding is that the way Union works, is that you create a game, and submit it to Union. Then, Unity will look for new and emerging platforms for your game. If they identify a market opportunity for your game, they'll strike a deal with the platform owner, and give the developers 80% of the revenue.

Addressing your point, it sounds like they don't handle iOS ports of games. From the FAQ page on Union,

We have no plans to place your games in the App Store. There are very few business barriers to placing games in the App Store, and no technical barriers using Unity. As such, we don't think we'd be providing much value by placing your games in the App Store for you.

Seems pretty cut-and-dry. They don't handle iOS ports of games.