The cost of armor is 10 gp per point of AC.
In older editions it was common to have the cost increase more rapidly for the higher-end armor, which created a dynamic of starting characters needing to save up for better armor, often a midpoint accomplishment between 1st and 2nd level that similarly increased their survivability. What fragments of historical price lists I can find online suggest a nonlinearly increasing price.
What was the intent of the straightline cost in ACKS?
In Moldvay/Cook’s B/X Set D&D, armor was price at 10gp per point of AC – Leather (AC7) cost 20gp, Chain (AC5) cost 40gp, and Plate (AC3) cost 60gp. B/X was the starting point for most ACKS prices.
I then checked them against historical references.
For middle ages prices, I assumed 1 silver piece in ACKS is worth 1 English silver penny, so an English pound is worth 24gp. Referencing some medieval prices, I saw some exorbitant prices occasionally listed for plate armor (Prince of Wales armor, 340L) but those were outliers. Complete lance armor is listed as L3 6s 8d in 1590 - roughly 80gp. Lance armor is listed as L4 in 1624 - roughly 96gp.
However, “complete lance armor” is a heavier and more expensive armor than ACKS Plate Armor. Note that ACKS states “historical examples include classical panoply (if worn with arm and leg armor), medieval plate and mail armor, Middle Eastern mirror armor, Eastern European plated mail, and Japanese tatami-do.” Full Renaissance plate might be AC8, or 80gp, in ACKS terms.
For ancient greek prices, I assumed 1sp is 1 drachma. The panoply of hoplite armor and weapons in Imperial Athens cost between 300 and 500 drachma, or 30-50gp. A panoply could consist of either linothorax (leather) or cuirass and greaves (less than plate - we’ll call it the equivalent of chain). It also included shield, spear, and short sword. So that gives us (20gp+10gp+3gp+7gp=)40gp to (40gp+10gp+3gp+7gp=)60gp. So ACKS is somewhat overpriced for panoply.
From this I concluded that pricing at 10gp per point of AC was not unreasonable, and quite within a range of realistic prices. Since it was also smooth and aesthetic, I stuck with it.
I suspect if you want to price more accurately, you actually should decrease the cost of leather, ring, and chain, rather than increase the price of plate.
(Hmm, it hasn’t been long enough for true thread Necromancy, but we’re beyond the range of most of the easily available cleric spells for restoring life, aren’t we?)
Where this ran into my suspension of disbelief is the Standard of Living table on page 39; looking at, say, http://www.jstor.org/stable/2596938, http://www.curufea.com/games/western/5e/wages.php, and http://www.newsfrombree.co.uk/m_econ.htm doesn’t leave me enlightened. The curufea.com site suggests that a semi-skilled laborer would be earning about 2 gp/month.
I should probably recalibrate my expectations, but it doesn’t help that I want a silver standard. Time to go read that thread. 
I believe what those websites are calling “semi-skilled labor”, ACKS calls “unskilled laborer”. In ACKS those professions earns between 3gp and 12gp per month, with a default presumption of 3gp per month. Peasants earn 1gp per month.