Go to Leonie, Foundry entrance merchant, she should have an option to smelt Metal Scraps that you get from recycling metal items. They will give you low quality steel.
a back and forth trip to foundry (in most cases) is 50 charons flat (1500 static value), the cost of processing 100 metal scrap (assuming they were all gained through loot instead of recycling more valuable items, which would make it at a loss) is 30 charons (900 value) and including a 30 minute wait time, it is next to impossible to save in these circumstances unless you looted all the metal scrap, walked to foundry, and have a very low mercantile skill. returning to the example of adrenaline, every medical vendor has the resources needed to craft it while standing in front of him while saving money; I think something as common as ammo or other combat utilities' crafting components should share the same characteristics.
How is spending 50 charons relevant to the topic? Especially when you can get to Foundry for free, and fast travel with Expedition for super cheap.
Aside from that, your OP stated:
as the game progresses, low-quality, cheap steel becomes harder to come by and, as a result, quantity producing crafting recipes such as caltrops, bullets, throwing knives, and bear traps increasingly lose cost savings until it's cheaper to just buy said bullets and utilities (unless you want a specific ammo type and/or poisoned utility), this has the effect that these recipies are almost not used at all.
For 30 charons you get low quality steel. If 30 charons is so much that it's impossible to save, then I have some bad news for you. You mentioned for some reason mercantile, well here is the math:
Since 30 charons = 900 value, we use x2 and x1.15 price values from mercantile, in this case we divide (because 900 is the final value).
450-250/7=28Q
&
782-250/7=76Q
This means for 30 charons at 0 Mercantile you can buy 28Q and lower. With discount cap Mercantile you can buy 76Q and lower.
Crafting ammo isn't meant as a substitute to buying, it's meant to give you additional resources, and even ammo hungry ARs and LMGs aren't gonna run out of either ammo or money with half-decent usage. You sell expensive loot, you recycle inexpensive, and with crafting get an additional stream of fuel for committing warcrimes on pipeworkers.
If you were able to save money and continue fighting just by crafting ammo alone, then there would be no point in the economy and skill system as it is. It doesn't take into account metal quality, which means you have to look at it under a different angle. For 30 charons and abundant Metal Scrap you can get MORE metal for crafting, not how much you are spending/saving. Mercantile just increases the amount you can buy for the same price raw.
This is ever more evident on DOM, where you sell items at 25% value, so you can't compensate dumping mags with one Tungsten Armor set sold and resupply at your local gun store.