Compiled from various posts on rec.games.frp.dnd.
Cost of a workshop to make only metal armor:
Cost of a workshop to make only leather armor:
Cost of a workshop for metal and leather:
Add costs of workers. Add 50% to cost for each additional 3 men over 3.
Also, chances that armor is too poor to sell, or commissioned person dies before it's ready, or piece made by apprentices won't sell.
Someone else added: 20'x10' is small for a shop. End cost is about 500-1500gp depending on terrain. 64 man-hours to rebuild if destroyed.
Remember that items are not kept on display, lest they be stolen, or taxes must be paid. Only an idiot would by weapons or armor not balanced and sized for them, except daggers and clubs.