- For PCs to experience the full range of the resource scale. It always irks me in OD&D that fighters of median starting money (or less) all get to start with plate mail, the best possible armor in the system; and thus have no room for improvement in that regard. It's much more interesting if they starting with a lower armor type, possibly leather, and work their way up through the options (even if just for a single adventure).
- To explain why so many NPCs in the world are just wearing leather armor. This would include the majority of bandits, brigands, etc.; if chain and plate are just marginally more expensive, then there would be almost no reason for them to not all be outfit in those heavier armors. Also, this armor-resource scarcity explains how thieves are able to masquerade as generic low-level fighters.
- To be somewhat more historically accurate. While many costs in OD&D, expressed in silver pieces (groats), are surprisingly close to their real-world historical documentation, one glaring exception is the armor costs. The lowest price I could find for historical plate documentation is about 200sp in our system, with most many times higher than that (see below for specifics).
Also, looking at the esteemed Medieval Sourcebook Price List (link; armor citation from Ffoulkes), we see the lowest-documents price for "Complete Lance Armor" at 3 pounds, 6 shillings, 8 pence (which converts for us as about 3×3×20+6×3 = 198sp). Other entries are at price points of 5, 8, or 16 pounds (or even 103 or 340 pounds for "gilt and graven" armor for the Prince of Wales in the 17th c.) -- so it's not unreasonable to set it possibly much higher.
What do 1st-level fighters get for those price points? Obviously, with starting money 30-180sp, plate mail is outside the budget for any starting fighters (and I think that's a good thing). Let's consider some "starting packages" with fighter budgets split into quartiles (1st Quartile meaning that 25% have this amount or less, etc.). In what follows, "sundries" means a backpack, waterskin, small sack, and three other 1-cost items (total 10sp).
- Poor Fighter (1st Quartile) -- Leather, helm, sword, shortbow, arrows, sundries (80sp). This is equivalent to a bandit archer figure. Alternatively, the fighter could trade the leather, shortbow, and arrows for chain mail; but then they wouldn't have any shield, missile, or backup weapon.
- Average Fighter (2nd Quartile) -- Chain, helm, shield, sword, light crossbow, quarrels (105sp). At this level, a character can definitely afford chain, but they can't get a pricey missile weapon at the same time; I also had to forego sundries in this list (hopefully assisted by other party members there).
- Rich Fighter (3rd Quartile) -- Chain, helm, shield, sword, shortbow, 40 arrows, sundries (130sp). With this amount of money, the fighter can comfortably purchase chain, as well as a nice missile weapon and extra ammunition, and whatever minor items they need.
Presumably the PCs are starting with middle-class type money, more than the peasant bandits who must do with only leather; although some unfortunate PCs may start at that reduced level of circumstance. Some few may start with only leather, helm, shield, sword, and sundries (55sp), similar to the majority of bandit light foot. On the other hand, fighters with above-average money can possibly start equipped as light cavalry: leather, helm, shield, spear, dagger, light horse, saddle, sundries (115sp) -- although obviously that's not optimized for a dungeon setting.
In summary: Changing the OD&D price points for armor hit that "proud nail" for me, and I don't see anything else on the original price list that bothers me the same way. It gives starting PCs reasonable kit-outs, and something to look forward to in the near future.