There's a rather good six-part series of blog posts that Bret Devereaux did about how iron and steel manufacture worked in the pre-modern world that can be found here: https://acoup.blog/2020/09/18/collections-iron-how-did-they-make-it-part-i-mining/

As part of it he notes that the investment the romans made into their legions was really quite incredible. Towards the end of part two he notes that the armaments of a legion of 5000 might amount to nearly 50 tons of iron, representing eighty thousand days of labour to make the charcoal alone. They completely deforested vast swathes of land to fuel their empire.

If we take a gladius to be 700g of iron, 3.5 of those tons would be in the swords. But a spear head isn't actually that much lighter. I've seen estimates of medieval spearheads at about half that. Let's say it's 300g, and you can save 2 tons of iron by going to spears. That's just 2 tons out of about fifty! Not really a huge saving in the grand scheme of things.