I've been combing through the things in my Player's Handbook that I normally gloss over and realized that there's not much distinction between studded leather armor and leather armor. The 5e manual states about studded leather armor:
Made from tough but flexible leather, studded leather is reinforced with close-set rivets or spikes. […] Weight: 13 lbs
And about leather armor:
The breastplate and shoulder protectors of this armor are made of leather that has been stiffened by being boiled in oil. The rest of the armor is made of softer and more flexible materials. […] Weight: 10 lbs
So to my understanding, either leather armor's boiling in oil is something that studded leather armor doesn't have, or the only difference between the two is about 3 lbs of rivets/spikes.
Comparing the price per weight to things like ball bearings, or 5 lbs of iron spikes, the rivets/spikes used in the armor should come out to about 1 gp.
If you had a party member who was proficient in leatherworking and was willing to donate their time, could you transform a 10 gp Leather Armor into a 45 gp Studded Leather Armor for a mere 1 gp?
Best Answer
Not by Rules as Written.
The Downtime Activities section of the DMG has nothing on creating mundane items, just magical ones.
The optional Downtime Revisited rules from Xanathar's Guide to Everything, however, have this to say (XGtE 128):
Therefore even if you counted Leather Armor as raw materials, you would only have 11 gold out of the required 22.5 gold.
The XGtE Downtime Revisited rules are optional, of course, so a DM could just rule that the plan works anyway, especially in a place like this, where it can be reasoned.
Additionally, below this is a table that states the tools required for various armors. It lists leather armor as something leatherworker's tools are capable of, and all other armors are made using smith's tools. It is unclear as to whether "leather armor" here applies to both types of leather armor or just the stat block of Leather armor, but I would rule that the creation of studs and rivets requires the use of smith's tools because of its metal nature.