The cost of plate now makes sense

I was reading Moldvay Basic this morning and I made a correlation that explained something. I had never understood why plate mail was so cheap (60gp) in B/X (and then BECMI and most of the clones) that a fighter will generally have it by second level with the normal treasure/creatures XP ratio and may have it as a starting character.

Here are the prices of armor in B/X:

Armor	Cost
Leather	20gp
Chain	40gp
Platemail	60gp
Shield	10gp

Which, when combined with AC gives us the following chart:

Description	AC	Cost	Cost per -1 from AC 9
Clothing only	9	0gp	0gp
Shield only	8	10gp	10gp
Leather	7	20gp	10gp
Leather and shield	6	30gp	10gp
Chain	5	40gp	10gp
Chain and shield	4	50gp	10gp
Platemail	3	60gp	10gp
Platemail and shield	2	70gp	10gp

The result is a nice, linear cost per gain in AC. This leaves me the option of finding true historical costs or creating my own mapping. Perhaps basing it on the square of the gain:

Description	AC	Cost	Cost per -1 from AC 9
Clothing only	9	0gp	0gp
Shield only	8	10gp	10gp
Leather	7	20gp	10gp
Leather and shield	6	30gp	10gp
Chain	5	80gp	20gp
Chain and shield	4	90gp	18gp
Platemail	3	180gp	30gp
Platemail and shield	2	190gp	27gp

Combined with my idea of awarding one XP per SP (and also reducing found treasure to 1/10 of the books list) that would mean a fighter would probably get platemail sometime between 3 and 4th level, which is consistent with my objectives.