There is a type of steel or iron, which acts similarly to copper - it builds up a coating of oxide (rust), the layer of oxide then protects it from further oxidation. Might it be the same material? The Angel of the North and various other outdoor things are made of it. It's called COR-TEN.
could be I have no idea, no idea how much that material Corten would cost, it was on sale with various other items at a Large General Garden Centre in Essex , near Chelmsford @ £69.99
I would imagine that its very cheap wholesale, Garden Centre would be making
£58 before VAT , and then maybe around 70-100% Margin - so maybe £30 wholesale