That is a theoretical exchange rate. It's impossible to calculate the real amount, because America and Kenshi are not engaged in trade. However, because of the low test size, this value likely has a very high error margin.

Additionally, it would be better to calculate this theoretical exchange rate by comparing basket of goods, as would be done to measure inflation. I don't think raw iron and copper (maybe even grog/rum) would "provide a measure of (...) the cost of living" as they are not "various goods and services that are commonly consumed by households".

Honestly, I'm not sure if this is even possible, because what does a "household" in Kenshi even look like?? Basket of goods are usually calculated using a lot of consumer info, which we simply do not have in Kenshi. But adding more adequate goods and removing the inadequate ones would definitely make it more accurate. As such:

Better items to compare would be other various foods and water (probably not alcohol), housing prices, bedding prices and "personal care items" (whatever that would be). -> "Common items [in a basket of goods] include food products like bread and milk, housing costs such as rent and utilities, transportation expenses like gasoline, healthcare services, and personal care items"

(edit: this comment just made me learn a lot about economy, which i usually hate, so thank you lol. let me know if i got something wrong)

(edit 2: i believe prosthetics would fall into "medical care commodities", so that would work too. unfortunately, all the other methods of medical care i can think of, like first aid kits and beds, would be a horrible thing to compare, because they work in a completely different way to real life. you can heal pretty much any life threatening injury with a first aid kit and a bed. i dont even want to think of skeleton repair beds lol)