> Stainless steel rebar is a thing. It's expensive […]

That's an understatement. Fiberglass (GFRP) tops out at roughly US$ 2.50/foot, while SS can get up to $9:

* https://www.wellcoindustries.com/the-cost-of-rebar-and-the-c...

The low-end of plain steel rebar is $0.40.

As the GP noted, it's still only a small bump in overall cost.

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